Losing mindfulness and peace after three weeks of successful practise

What do I have to do to restore MY mindfulness?

Maybe the highlighted is the root cause. As long as you still see the "I", "mine", "myself" involved in moments of anger, restlessness, and even peace, there'll never be a state of real peace. Drop the "I", "mine", "myself", simply observe the arising of anger, restlessness, AND of peace, then real peace will come. Sorta like you're trying to chase your own shadow. The harder you try to chase after it, the more it'll run away from you. But the moment you stop and settle down, it'll also stop and stay with you!

It seems the mind did not reach neighborhood concentration, let alone jhana. This being so, what of value was lost? Nothing. Since nothing of value was lost, what is there to restore? Nothing.

In one sutta, the Buddha said to "rely on nothing". Since there was & is nothing, it seems this state of relying on nothing has already been reached.

“Looking to nothingness and being mindful, Upasīva,” said the Gracious One, “depending on nothing, cross over the flood.

Snp 5.7

The word "mindfulness" ("sati") means "to remember" or "to bring to mind" the Dhamma Teachings (SN 46.3 paragraph 2; MN 117 paragraph 9). Therefore, mindfulness brings to mind different teachings or different wisdoms to use as an antidote for different situations.

The appropriate wisdom or antidote for a specific situation is called "sampajāna". Therefore, in Buddhism, the terms "sati-sampajāna" ("mindfulness-&-clear comprehension") are often found used together because, in reality, one cannot exist without the other.

In your case, when anger or restlessness arise in your mind, if you are unable to return easily to the meditation object of breathing, you should then be mindful of the relevant Buddhist Teachings about how to overcome anger & restlessness.

In summary, "mindfulness" is not used in only one specific way (such as for watching breathing).

More information is here: The Natural Cure of Spiritual Disease.

The root of distraction, anger, and restlessness is attachment to some idea or some value that you assign to something. You identify with this value and think it makes you who you are. This habit of holding to such fragments is called I-making. You need to learn to let go of who you think you are. When you let go of rigid opinions - you let go of the basis of disturbing emotions. You lose shape and become nobody. Then your meditation can be perfect whether you sit or walk.

Some good advice given by Ajaan Fuang in "Awareness Itself":

§ "When the meditation goes well, don't get excited. When it doesn't go well, don't get depressed. Simply be observant to see why it's good, why it's bad. If you can be observant like this, it won't be long before your meditation becomes a skill."

§ "Everything depends on your powers of observation. If they're crude and sloppy, you'll get nothing but crude and sloppy results. And your meditation will have no hope of making progress.

And also others:

§ A young nurse practiced meditation with Ajaan Fuang several days running, and finally asked him one day, "Why wasn't today's session as good as yesterday's?"

He answered: "Meditating is like wearing clothes. Today you wear white, tomorrow red, yellow, blue, whatever. You have to keep changing. You can't wear the same set of clothes all the time. So whatever color you're wearing, just be aware of it. Don't get depressed or excited about it."

§ A few months later the same nurse was sitting in meditation when a sense of peace and clarity in her mind became so intense that she felt she would never have a bad mood infiltrate her mind again. But sure enough, bad moods eventually came back as before. When she mentioned this to Ajaan Fuang, he said, "Looking after the mind is like raising a child. There will have to be bad days along with the good. If you want only the good, you're in for trouble. So you have to play neutral: Don't fall in with the good or the bad."

§ 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 seamstress, after practicing meditation with Ajaan Fuang for several months, told him that her mind seemed more of a mess than it was before she began meditating. "Of course it does," he told her. "It's like your house. If you polish the floor every day, you won't be able to stand the least little bit of dust on it. The cleaner the house, the more easily you'll see the dirt. If you don't keep polishing the mind, you can let it go out and sleep in the mud without any qualms at all. But once you get it to sleep on a polished floor, then if there's even a speck of dust, you'll have to sweep it away. You won't be able to stand the mess."

§ "Persistence in the practice is a matter of the mind, and not of your posture. In other words, whatever you do, keep your mindfulness constant and don't let it lapse. No matter what your activity, make sure the mind sticks with its meditation work."

§ "Meditating isn't a matter of making the mind empty, you know. The mind has to have work to do. If you make it empty, then anything — good or bad — can pop into it. It's like leaving the front door to your home open. Anything at all can come strolling right in."

All of those things are coming back because you haven't really fixed anything. So many people use meditation as a vacation. They run themselves ragged in their day to day lives only to use the cushion as a kind of karmic hospital. This is palliative practice and just like a vacation from your job or your terrible marriage, spending a week in the zendo or Tahiti isn't going to do a damn thing for your mind. Meditating this way isn't any different from popping pills or smoking a blunt. Sure, your problems melt away for a time, but you aren't addressing the crap that made you want to melt them away in the first place.

We sit to prepare our minds for insight. Anything else that happens - jhana, bliss, IDK talking to devas - all that is wholly ancillary. Rather than try to preserve those feeling of bliss, wholeness, and peace, instead turn that bright and less blemished mind to your own personal obstacles. There are entire suites of insight, whole canons of material that you can investigate from Zen koans to vipassana objects, etc. Gaining direct knowledge into those things are what's going to stop those bad habits from reemerging. Simply sitting isn't going to cut it. Bliss doesn't change anything.

And for what it's worth, you're just starting. Of course you are going to hit a roadblock. I've been doing this stuff for the better part of two decades and I still have good practice days and bad ones. Keep at it. It'll come back.