being a painter as well as a mapper, i'd have to say it's an artist thing... i find especially with my paintings... i HATE them... in pretty much every way, i find things didn't turn out good enough... Something I've found is fairly successful is to just take a break... for me it seems to be the longer i take away from the painting, the more i like it when i look at it again... I think we just get fixated on the piece, and see all it's flaws.. which no one else ever sees... taking time away allows me to forget what it looks like, what i was striving for, and to see it how i would see another artists work... try it! you'd be surprised how a little makes a big difference!

And also remember, no one is as hard on your art as you!