Do this, now if you can; soon if you need later.
Start writing down your successes: those times when you impressed, the guesses you got right, the long shots that made it into the hoop. You have them, many of them over your years. The times you could see approval in the eyes of your friends, interest in a small group after you spoke, adulation from a child.
Write these down. Yes, there ARE many. Think of them. Sketch out the scenes, the dialog. You are the main protagonist. It doesn’t have to be a glowing story, though it may turn into one. Just write it down!
Come back at it again and write some more occasions, or fill in detail from the ones already captured. Get a bunch of this down. Then, next time you don’t feel so hot about yourself, next time you screw up and wonder if you’re hopeless, read them over. Remember the times you excelled, you shined, you blew them away!
Your current funk is temporary, your self-questioning is false, you are worthy and will be back on top of your game again. Then rest, and let your unique energy renew, readying yourself for the next challenge.