I'm convinced that depression is not a real thing and I instantly lose all respect for people who claim to have it
That is a bold claim. What makes you think that depression isn't real?
Because when I'm depressed I find people who suffer from it unbelievably annoying.
Well seeing that you just said "when I'm depressed" you do think it's real. You shouldn't be so careless as to say depression isn't real and losing respect for people who are suffering from is despicable. It sounds like something happened to you and you're lashing out so maybe talking to someone can help resolve those feelings
You make your own problems by thinking so much.
I think that's the problem here, you shouldn't downplay people who have depression because it is extremely hurtful to invalidate their illness. Since you seem to have it yourself, how about trying therapy to sort out all of this overthinking and have a clearer more positive opinion?