"I'm not good enough."
Those four little words are like a personal plague, something that chews me up from the inside out. I know I'm not the only one: I've been in any number of creative hobbies over the years, and that sentiment twisted through them like poison vapor. It's always there. Once you learn enough about something to realize how much there is to learn, how much you don't know, it can be crushing. If your self-confidence is weak to begin with, it could be too much, you might just give up - after all, you'll never be good enough.
I'm not a very confident person, truth be told. If I don't correct myself I'm shy, I'm quiet, and I have a tendency to wait so long for everyone else to have their say that I end up saying nothing. (I suppose I'm living proof of every warning you've ever had about how it's always the quiet ones that are... well, in my case, secretly a filthy pervert. But I digress.)
In short, I don't believe that I'm good enough, and maybe I never will. I deal with it. Self-doubt doesn't mean that I should give up, it means that I should try to prove myself wrong. The worst I can do is to meet my expectations, after all. Maybe some people are just naturally self-confident; I know that I'm not, but that doesn't mean the rest of the world has to know. You're the only one who knows what you think of yourself.
My point here is that even if you feel like a hack, a dunce, like all you've made belongs at the bottom of the Marianas Trench and that even there the eyeless bottom-feeders will bump into it, in the true blackness, and recoil in disgust... don't believe yourself. Keep going. Keep trying! You can only improve if you don't quit, and simple perseverance can keep you a step ahead.
You're not the best yet, but everyone starts somewhere. Give yourself a chance.