i keep playing games im really bad at, which i like watching from time to time but only make me feel like a dumbass cos i lose costantly (mainly chess).
after that, i tell myself im good for nothing, and i go back into a depressive/anxious spiral that can last half a day if i dont manage to calm myself down, but even after that i try to try again the day after, which one might think is the good and honest thing to do, right? like trying again to try to improve? but then i lose another couple of games/do badly at the videogamez and it restarts all over again and i cry on myself again. i know it might sound like im exaggerating but i think it's some sort of self harm.
and im starting to doubt if im good at anything, cos i really am not. like the only thing i've been more than ok at for the past two years or so is my high school math, which, tbh, is going great, im helping out basically all my friends pass, but even slightly outside of that i am complete trash at stuff, like not even worth trying.
I think everyone has already given really good advice, but I had one thought to add. Sometimes, when we get stuck in compulsive, repetitive behavior like this, it's because our brain is trying to resolve something. Usually taking a story from our past and building a different ending. So maybe in your case you failed at something, and the consequences were really dire, and now when you fail again your brain punishes you to try and keep it from happening. Or maybe you had horrible consequences from being overly confident and so your brain is trying to remind you never to do that again. Something like that.
One way out of stuff like this is to change the ending. So in your case, it seems like you could try and deliberately reward and comfort yourself after the next time you lose at a game. Maybe plan out in advance a gift or favorite food you could give yourself once you lose, or schedule a time to immediately sit down and talk with someone who loves you. Write down some affirmations of self-compassion and force yourself to look in the mirror and say them. Talk to yourself like a loving parent would speak to a child who messed up on something. It will definitely feel silly and weird to do these things, but whatever your reaction is, it'll hopefully give you useful information on why this sort of thing is happening.