A few things.
1) This does not make you a kofer, ch”v, because you don’t deny the concept of reward, you just (likely erroneously) don’t think you will get any. You do acknowledge that others will, so you’re not denying s’char v’onesh.
2) You do get s’char for the mitzvos you do, even if others may get more because it’s harder for them. It’s not a competition, and Hashem is not limited in hows much s’char He can give.
3) There is indeed no reward for not committing an aveirah which you had no interest or desire to commit.
I hope you are getting help for your depression.