How to maintain a Buddhist practice through dark times?

I guess you are having problems due to discontentment, it's difficult to figure out the particulars because you don't say much about what issues you are having. Here are some Sutta excerpts you might find helpful;

[Kamada:] So hard it is to do, Lord, It's so very hard to do!

[Buddha:] But still they do what's hard to do, Who steady themselves with virtue. For one pursuing homelessness, Content arrives, and with it joy.

[Kamada:] So hard it is to get, Lord, This content of which you speak!

[Buddha:] But still they get what's hard to get, Who delight in a tranquil mind. The mind of those, both day and night, Delights in its development.

[Kamada:] So hard it is to tame, Lord, This mind of which you speak!

[Buddha:] But still they tame what's hard to tame, Who delight in senses at peace. Cutting through mortality's net, The nobles, Kamada, proceed.

[Kamada:] So hard it is to go, Lord, On this path that gets so rough!

[Buddha:] Still nobles, Kamada, proceed On paths both rough and hard to take. Those who are less than noble fall On their heads when the path gets rough. But for nobles the path is smooth — For nobles smooth out what is rough! https://www.accesstoinsight.org/tipitaka/sn/sn02/sn02.006.olen.html

At such times, monks, as the mind is sluggish, that is the wrong time to cultivate the enlightenment-factor[1] of tranquillity, the enlightenment-factor of concentration, the enlightenment-factor of equanimity. What is the reason? A sluggish mind is hard to arouse by these factors. [...] "But, monks, when the mind is sluggish, that is the right time to cultivate the enlightenment-factor of investigation-of-states, the enlightenment-factor of energy, the enlightenment-factor of rapture.[2] What is the reason? A sluggish mind is easy to arouse by these factors. https://www.accesstoinsight.org/tipitaka/sn/sn46/sn46.053.wlsh.html

“Bhikkhus, possessing three qualities, a bhikkhu is practicing the unmistaken way and has laid the groundwork for the destruction of the taints. What three? Here, a bhikkhu guards the doors of the sense faculties, observes moderation in eating, and is intent on wakefulness.

“And how, bhikkhus, does a bhikkhu guard the doors of the sense faculties? Here, having seen a form with the eye, a bhikkhu does not grasp its marks and features. Since, if he left the eye faculty unrestrained, bad unwholesome states of longing and dejection might invade him, he practices restraint over it; he guards the eye faculty, he undertakes the restraint of the eye faculty. Having heard a sound with the ear … Having smelled an odor with the nose … Having tasted a taste with the tongue … Having felt a tactile object with the body … Having cognized a mental phenomenon with the mind, a bhikkhu does not grasp its marks and features. Since, if he left the mind faculty unrestrained, bad unwholesome states of longing and dejection might invade him, he practices restraint over it; he guards the mind faculty, he undertakes the restraint of the mind faculty. It is in this way that a bhikkhu guards the doors of the sense faculties.

“And how does a bhikkhu observe moderation in eating? Here, reflecting carefully, a bhikkhu consumes food neither for amus*m*nt nor for intoxication nor for the sake of physical beauty and attractiveness, but only for the support and maintenance of this body, for avoiding harm, and for assisting the spiritual life, considering: ‘Thus I shall terminate the old feeling and not arouse a new feeling, and I shall be healthy and blameless and dwell at ease.’ It is in this way that a bhikkhu observes moderation in eating.

“And how is a bhikkhu intent on wakefulness? Here, during the day, while walking back and forth and sitting, a bhikkhu purifies his mind of obstructive qualities. In the first watch of the night, while walking back and forth and sitting, he purifies his mind of obstructive qualities. In the middle watch of the night he lies down on the right side in the lion’s posture, with one foot overlapping the other, mindful and clearly comprehending, after noting in his mind the idea of rising. After rising, in the last watch of the night, while walking back and forth and sitting, he purifies his mind of obstructive qualities. It is in this way that a bhikkhu is intent on wakefulness.

“A bhikkhu who possesses these three qualities is practicing the unmistaken way and has laid the groundwork for the destruction of the taints.” https://suttacentral.net/an3.16/en/bodhi

There is the case where a monk remains focused on unattractiveness with regard to the body, percipient of loathsomeness with regard to food, percipient of non-delight with regard to the entire world, (and) focused on inconstancy with regard to all fabrications. The perception of death is well established within him. He dwells in dependence on the five strengths of a learner — strength of conviction, strength of conscience, strength of concern, strength of persistence, & strength of discernment — [...] he attains [...] the immediacy that leads to the ending of the effluents. https://www.accesstoinsight.org/tipitaka/an/an04/an04.163.than.html

"Endowed with seven qualities, a monk pursuing mindfulness of breathing will in no long time penetrate the Unprovoked [release]. Which seven?

"He is a person who imposes only a little [on others]: one of few duties & projects, easy to support, easily contented with the requisites of life.

"He is a person who eats only a little food, committed to not indulging his stomach.

"He is a person of only a little sloth, committed to wakefulness.

"He gets to hear at will, easily & without difficulty, talk that is truly sobering & conducive to the opening of awareness: talk on modesty, contentment, seclusion, non-entanglement, arousing persistence, virtue, concentration, discernment, release, and the knowledge & vision of release.

"He lives in the wilderness, in an isolated dwelling place.

"He is a person of much learning, who has retained what he heard, has stored what he has heard. Whatever teachings are admirable in the beginning, admirable in the middle, admirable in the end, that — in their meaning & expression — proclaim the holy life that is entirely complete & pure: those he has listened to often, retained, discussed, accumulated, examined with his mind, and well-penetrated in terms of his views.

"He reflects on the mind as it is released.

"Endowed with these seven qualities, a monk pursuing mindfulness of breathing will in no long time penetrate the Unprovoked."

If you want i can give more info explaining these.

My only motivation is Nibbana, nothing else

You have the right goal. This Dhamma expounded by the Buddha is only for that goal and that goal only. If one seeks this Dhamma for any other reason, for debate, for advancement of intellect, etc., they have grabbed a great snake from the tail or the body and therefore will suffer death or deadly pain. MN22

The question you should ask yourself is, "Am I in the Path to Nibbana"? From your symptoms it sounds like you have taken a different path than the Path Leading to the Cessation of Suffering

In Buddha's dispensation, Right Concentration is the concentration that is equipped with seven factors - Right View, Right Thought, Right Speech, Right Action, Right Livelihood, Right Effort and Right Mindfulness. MN117

Today the Buddhist meditations have been lost to the world and instead have been replaced with breathing concentration that doesn't lead to Right Liberation.

Buddhist meditations are about contemplating on Dhamma, reflecting on your own self to see the Dhamma within you. Then you see the what the Buddha meant through your own experiences. And what you realize is a link in the dependent origination through your own experience.

The start of the Noble Eight Fold Path is by developing the (the Noble) Right View. For that, you have to contemplate on Suffering, the Origin of Suffering, the Cessation of Suffering and you will be back again in the Noble Eight Fold Path as the path leading to the cessation of suffering.

When you practice this way, what you gain is the unwavering Faith (saddha) in the Buddha. He knows within that this Noble Eight Fold Path is the Right Path and no matter what others say you are determined to Develop this Path. The Noble Eight Fold Path is to be developed. There are Four Developments in the Noble Eight Fold Path. Namely Development of Body, Development of Virtue, Development of Mind, and Development of Wisdom.

That is the Path you need to take to attain Nibbana. For a person taking this Path, he is never lost; never has dark times because he is always with the Buddha, Dhamma and the Noble Sangha.

Listen to this great explanation of the Noble Eight Fold Path and try the meditation exactly as explained. See if that makes a difference for your current situation.

With Metta

There is lot more benefit from sincerely observing precepts, giving into generosity and tune ones life, while being all around observant, especially on reflection, rather the foolish modern compensation meditations, even in retreats, for householders. Get engaged in services in monasteries and try to fix the issue of gross defilements! To be very straight forward. If physical locked down, good housholder, there might be ways out there but so far my person coild offer such only here with plenty of work and task, done to be with Sila, as kammatthan, all around the Gems, headed toward Nibbana, an allbaround Vipassana and awakening camp...

[Note that this isn't given for trade, exchange, stacks or what ever binds here, but to escape from it]

While sitting meditation is good, it shouldn't be your sole activity of cultivation. Obviously a sick and frail body won't get one very far. You'd need a strong healthy body to take on the task too. So don't forget your daily physical exercise regime. The best ones are outdoor cardios like jogging, biking, hiking, etc. Also try to incorporate some walking meditation to your training, not just sitting meditation. Remember, if you don't take care of your physical body, you'd basically open up the door for Sloth-and-torpor/thīna-middha and Restlessness-and-worry/uddhacca-kukkucca, 2 of the Five Hindrances to invade and hinder your progress.

I am in your state. I am sick as well. I want to be a monk but I can't.

The difference is I don't feel bad because I can see my effort when looking back to check what I did.

I can say I never sit as much as you, and I can say if I can practice as much as you doing, I am going to be very happy and proud of it surely.☺

I understand the chance to meet Dhamma is extremely difficult, every practitioner doesn't want to miss this chance. However, you are trying and you still not giving up, so don't count your chickens before they hatch. 😉

Samatha is for happiness, Vipassana is for serious considering. Both are helping each other, but Worry is for lost.

Relax. ❤♡♥