>>299134483
Is it moving you backwards when you hold your thumb on a thumbstick, or only after you'll let it off? If it's after you let it off, it might just be some dirt: thumbstick rebounds back, moves a tiny bit backwards from the spring motion, and gets stuck in the opposite position. Wiggle it around a lot, be a bit rough with it, pull it a bit while moving it from side to side, check if you can clean it somehow, see if it helps.

If it moves you backwards when you move it forward and you're sure that you're actually moving it forward, it's probably a different issue.