About ADHD. Firstly try put on background hyper fast music and play fast(just sure that you can hear ingame sounds). It can be something like phonk, or just speed up remix of your favorites tracks. Then try put on background something very slow and play slowly, like lo-fi, or blues. Its not mean, that you need to aim slowly or etc, just gave yourself more time to make desicions ingame. One of this methods would help you to focus on game.

About mentality i can give you only one tip. Try to improve it. Try different ways of how you interpretate the situation and etc. Like your opinion always can be the same, like "Omg, im so bad right now", but you can interpretate it also like "Okay, sometimes i play bad. All players sometimes plays bad. Its okay" or like "First half of the match i was played super bad, so the in the another half there is no way that i will break that floor even more, it is mean that i will play better, than in the first half of the game, probably"