Oh god, it's so easy to doubt yourself! It doesn't matter how skilled you are, you'll always doubt yourself. Which is horrible- but also a good thing?
I haven't seen anyone here else mention it but--
1) Remember you are your own worst critic. You will ALWAYS see things that can be improved on in your own art, and see blaring flaws. Unlike when you look at other people's artwork. After all YOU'VE probably just spent the past 12 hours looking intently at your own art.
Another thing is-- remember your artist eye!! Not only as an artist are you developing your drawing skills, but your seeing skills. Sometimes you're better at seeing something correctly than drawing it, and vice versa. But that's how you improve!! Be proud of that!!
At least-- these are things I like to remind myself of when I'm feeling super down about my art.