How does one keep oneself motivated to follow the path of dhamma?

The Buddha said:

AN4.162:1.1: “Mendicants, there are four ways of practice. What four? Painful practice with slow insight, painful practice with swift insight, pleasant practice with slow insight, and pleasant practice with swift insight.

So if practice is painful and slow the question then becomes...why not choose the pleasant practice?

AN4.162:5.1: And what’s the pleasant practice with swift insight? It’s when someone is not ordinarily full of acute greed, hate, and delusion. They rarely feel the pain and sadness that greed, hate, and delusion bring. These five faculties manifest in them strongly: faith, energy, mindfulness, immersion, and wisdom. Because of this, they swiftly attain the conditions for ending the defilements in the present life.

Acute greed, hate and delusion are painful, so we get rid of them. In fact, getting rid of greed, hate and delusion allows faith, energy, mindfulness, immersion and wisdom to grow. It's definitely more pleasant.

Consider the three practice categories: wisdom, ethics and immersion. People tend to skip the first two and fast forward to IMMERSION. I did. It worked for a while. Then it became unpleasant and slow. So I started to practice wisdom and ethics. And after a while, immersion became pleasant again.

Read the suttas and explore wisdom. We all think we're wise. But we're not:

MN44:11.5: Right view and right thought: these things are included in the category of wisdom.”

Read the suttas and explore ethics. We all think we're ethical. But we're not:

MN44:11.3: Right speech, right action, and right livelihood: these things are included in the category of ethics.

At some point, you'll want to meditate:

MN44:11.4: Right effort, right mindfulness, and right immersion: these things are included in the category of immersion.

At that time, it will be pleasant and fast!

In particular, if you don't want to be reborn, find out what you're holding on to and let it go. That takes wisdom and ethics and immersion.

I think a lot of people come to the dharma with the idea that they are going to sit down on the cushion, drop into deep concentration, and that suddenly all of their problems will be washed away by some intoxicating wave of bliss. Were it that easy! The truth of the matter, the reality, is that progress in Buddhism is gradual, almost imperceptible. While it follows a set pattern (like what you'd find in the Samannaphala Sutta), it only looks like that from the outside and in retrospect. The first hand view of Buddhist practice is marked by constant doubt, sweat, discouragement...and an occasional parting of the clouds. Our struggle is far more real to us that the maturation of our minds. In fact, we only stand a chance at becoming a Buddha when our experience of our own ignorance thoroughly convinces us that we could never become one. If you feel hopeless, lost, and inextricably bound to samsara, good, you are headed in the right direction!

But please take heart - you would only know to ask a question like this if you were actually making progress. We don't face difficulties unless we are moving forward and away from the habits, ideas, and perceptions that characterized our untrained mind. This phase in your practice could be a real turning point for you as it asks you to address a very fundamental problem that many people bring to the cushion - namely the idea of "progress", and it's fundamental unreality. Buddha's aren't made, they simply are. You can't progress toward something that isn't a destination. The Buddhist path isn't transactional like a bank account is. We don't deposit our effort checks at Shakya First Republic and expect to withdraw them at a later date, with interest.

The first paramita is dana, or geneorsity. It is characterized by a giving that expects no reward, an effort that has no result. In order to practice the great way, it's up to you to make a complete offering of yourself to the Buddha, Dharma, and Sangha with no precondition of your own enlightenment. Be willing to accept failure after failure, lifetime after lifetime with no hope of ever waking up. This is the true heart of faith, the trusting mind that is the road to Buddhahood. Take your first steps into a dark room. Proceed from the top of the flag pole.

https://youtu.be/xFntFdEGgws

When I go to the doctor and he does a blood pressure reading, sometimes I am anxious about how the reading will turn out. This anxiety in turn makes the reading go higher, which is not what I wanted in the first place. To get a good blood pressure reading within a good range, I need to let go of my anxiety and expectation.

It's pretty much the same with regards to doubt and your practice.

More on doubt below ...

There are the five hindrances which can prevent you from progress in your practice. They are: sensory desires, ill will, sloth and torpor, restlessness and remorse, and doubt.

What you're experiencing here is doubt - "can I do this?", "is it possible?", "where am I going with this?", "is this the right way?".

The general advice for overcoming the hindrance of doubt is letting go and trusting the inner silence (see quote below).

Here's Ven. Ajahn Brahm's advice on overcoming the hindrance of doubt, from this essay:

Doubt refers to the disturbing inner questions at a time when one should be silently moving deeper. Doubt can question one's own ability "Can I do This?", or question the method "Is this the right way?", or even question the meaning "What is this?". It should be remembered that such questions are obstacles to meditation because they are asked at the wrong time and thus become an intrusion, obscuring one's clarity.

The Lord Buddha likened doubt to being lost in a desert, not recognising any landmarks.

Such doubt is overcome by gathering clear instructions, having a good map, so that one can recognise the subtle landmarks in the unfamiliar territory of deep meditation and so know which way to go. Doubt in one's ability is overcome by nurturing self confidence with a good teacher. A meditation teacher is like a coach who convinces the sports team that they can succeed. The Lord Buddha stated that one can, one will, reach Jhana and Enlightenment if one carefully and patiently follows the instructions. The only uncertainty is 'when'! Experience also overcomes doubt about one's ability and also doubt whether this is the right path. As one realised for oneself the beautiful stages of the path, one discovers that one is indeed capable of the very highest, and that this is the path that leads one there.

The doubt that takes the form of constant assessing "Is this Jhana?" "How am I going?", is overcome by realising that such questions are best left to the end, to the final couple of minutes of the meditation. A jury only makes its judgement at the end of the trial, when all the evidence has been presented. Similarly, a skilful meditator pursues a silent gathering of evidence, reviewing it only at the end to uncover its meaning.

The end of doubt, in meditation, is described by a mind which has full trust in the silence, and so doesn't interfere with any inner speech. Like having a good chauffeur, one sits silently on the journey out of trust in the driver.

Also see his conclusion:

Any problem which arises in meditation will be one of these Five Hindrances, or a combination. So, if one experiences any difficulty, use the scheme of the Five Hindrances as a 'check list' to identify the main problem. Then you will know the appropriate remedy, apply it carefully, and go beyond the obstacle into deeper meditation.

When the Five Hindrances are fully overcome, there is no barrier between the meditator and the bliss of Jhana. Therefore, the certain test that these Five Hindrances are really overcome is the ability to access Jhana.

Ven. Ajaan Fuang Jotiko gave a shorter version of the solution to overcoming the hindrance of doubt in "Awareness Itself":

A student came to complain to Ajaan Fuang that she had been meditating for years, and still hadn't gotten anything out of it. His immediate response: "You don't meditate to 'get' anything. You meditate to let go."

The answer to your inner doubts is ... just let it go!

Does it matter whether you gain Nirvana is 10 months or 10 years or 10 lifetimes? Is there a deadline?

Just keep practising with heedfulness, as the Buddha stated in the Mahaparinibbana Sutta:

Then the Gracious One addressed the monks, saying: “Come now, monks, for I tell you all conditioned things are subject to decay, strive on with heedfulness!” These were the last words of the Realised One.