Tis the curse of a perfectionist I'm afraid, always self critical and never totally happy with your own work. Saying that, if I squint my eyes whilst looking at it, it looks near as dammit perfect, so I'll settle for that or else I'll drive myself bonkers!
I think we may suffer from the same nail OCD. I see errors in my nails that others don't, but if I'm not happy I just have to redo it, or it bugs me all day, lol.
I was thinking.... Have you thought about using stripping tape? It would be a little tricky and you would have to plan out how you would layer the colours but you could end up with perfect straight lines.
Here are a few links:
Chalkboard Nails: Spider-mani: The Metallics
Makeup Withdrawal: Striped Manicure
http://24.media.tumblr.com/tumblr_m0fp4q3n6C1rqjazvo1_1280.jpg