I keep my old art around.
If I ever feel like I'm losing confidence and am going nowhere with my art and not improving - I pick out a sketchbook, or open up a .PSD, of something I drew more than a year ago, and compare it with something I've recently drawn. 99.99% of the time, I see a vast improvement. Sometimes you improve in leaps and bounds, and sometimes you improve slowly, bit by bit - and it's the bit-by-bit type of improvement that's easy to get discouraged about, because it's so hard to tell it's happening.
Sometimes, you need to take a break, because the stress is getting to you, but I've found that the only way to keep from losing confidence in something is to keep doing it. Keep drawing. Accept the fact that a lot of what you're drawing is going to be crap - and is always going to be crap. There will be warm-up sketches you throw away the moment you finish them (or if you're me, keep around to remind myself of my progress), there will be serious drawings that looked amazing in your head but look impossible on the page; there will be more misplaced noses, wonky hands and misaligned eyes than you can shake a stick at.
The important thing to learn is that this is perfectly okay. Sometimes, you have to draw all the wrong stuff just to get it out of the way so that you can draw stuff right.
Everything you draw is turning out terribly, take a deep breath and remind yourself that it's okay. You can do better with the next drawing. And the next one after that. And the next one after that.