Any artist/musician/creator/human-being knows the feeling - pure, unfiltered frustration over hours of hard work gone to waste on something that undeniably looks like crap. Over time, I have definitely become better at accepting these frustrations, perhaps because these fits of artistic disappointment in myself occur so frequently, I have no choice but to accept them. Through practice, it has become a lot easier to see each crumpled paper or discarded sketch as a learning opportunity rather than as a pointless mistake, and I know that no effort goes to waste when it comes to improving my technical skill. That being said, there is no remedy for that tug at the bottom of your stomach when a painting or a picture doesn't come out quite as well as you pictured it in your head.
A related feeling also familiar to artists/musicians/creators/humans is inadequacy. Not being "good enough", whether that is "good enough" for yourself or for someone else, in whatever it is you're trying to be good at is pretty crushing. This used to haunt me a lot in my art-making process. I went to a nice suburban high school with an excellent visual arts program, and I was definitely not the best artist, or the second or the third best (not even top ten if we're being honest), in my graduating class. When I graduated, I wasn't entirely confident in my decision to major in Art Ed., not because I didn't think I'd love it, but because I wasn't sure if I would be good enough. I now go to a university of over 50,000 undergrads, attend classes with extremely talented individuals and live in a city with amazing artists and a thriving art scene. I've never been the best, and its hard to tell if I'm ever even just "good enough", but I could not be more happy and confident with the choices I have made for my future. A lot of practice, patience and professors have helped change my views of how I approach the traps of comparing myself and my art to others, but one thing that has stuck with me was this video I saw about a year ago. (skip to 0:40 for the cool part)
Lately it has been extremely hard for me to feel good enough for just about anything, but I dug up this video and its helped. I'm going to try to keep it in mind the next time I take out the trash filled with crumpled-up sketches or lug a twenty pound wire sculpture mess out to the dumpster.