I think patience and a keen eye for detail really pays a lot in making artworks like these. By the end of the day (weeks or months) you can look at your work and say, "Well, aren't you a beauty?" or "Did I really do this?"
I get that feeling a lot. I begin with the sketch then I lay down some flat shading and I know that if I look at it in full view I'm going to be disappointed. But if I keep my head down and just go through it bit-by-bit I'll eventually look at the whole thing and surprise myself. There is a point where my artwork looks super-ugly. I just have to suck it up and keep going.