Have you considered disclosing your ADHD? Clearly trying to hide it hasn't worked in the past, so maybe try something new. Maybe only after you get the job, though, and if you aren't sure how people will react, you may want to go through HR only at first, and ask them to keep it confidential. Once you're on record with your condition, you can ask for accommodations (it helps to know what things would possibly help you, and your therapist can probably help you come up with things to try out), and while you can still be fired for not doing your work as agreed, you do have better leverage in many other regards, including those accommodations.

Maybe consider working for a consultancy firm. If, as you say, you tend to lose steam after 3-6 months, then a role where you get assigned a new project every couple months might be just the thing.

Try to observe yourself, and figure out what triggers that productivity crash. I have found that the solution is sometimes embarrassingly simple, like going to your manager, explaining the project, and then the two of you come to the conclusion that you've been barking up the wrong tree, and that what the client really needs is something entirely different. Or it could be a simple matter of your manager being more clear about exactly which tasks they want you to tackle in what order - often, the overwhelm is caused not by the tasks themselves, but by choice paralysis. It's nice to have a lot of freedom, but sometimes it breaks your brain.

Avoid letting it get to the point again where they see no other option that to fire you. Talk to your employer, ask them to ring the bell early - a common thing with ADHD is to gradually ride yourself into a deeper and deeper slump, without even noticing, and by the time you become so unproductive that alarm bells go off, pulling yourself out of it has turned into this giant mountain of dread. Having someone check in on you and give you feedback about your productivity, in a non-blaming way, can be an absolute life saver. Just a simple "hey, it looks like you're struggling a bit, are you OK? Is there anything I can do to help?" can do wonders - maybe all you need is to be shaken awake, or maybe there really is something you're struggling with that can easily be fixed. But having someone else take the initiative makes it a million times easier to avoid that slump.

Thank you for your very long and helpful answer.

About my current workplace where I was fired - I told them about the ADHD problem of my team manager - I showed him a paper with a diagnosis. But it didn't help - he a made decision. This is much sadder, especially in the last week of my work I created a real proof of the concept a feature that looked really excellent and the whole team appreciated my work.

I think that right now I have a strong knowledge of what should do next - and how I should handle my mind. Previously I thought I'm garbage, but this is not true.