Aaw man, I'm in the middle of doing a redesign too and am in the same boat as you. Sometimes I can see a design really clearly and know how to lay it out, but most times it's not clear and I have to tweak and tweak and tweak it until finally it turns into something I'm satisfied with. Then on top of that I'm not the greatest when it comes to CSS, so I might know how I want it to look, but it takes me hours trying to learn the CSS I need to know to get it done.
I have scrapped quite a few designs completely because it has just gone too far from what I originally wanted and I lost my way. I hate trying to decide on colors, that's my least favorite part.
Anyway, I know how you are feeling right now. Good luck and I look forward to seeing what you come up with.