It is comforting to know that this is a common problem. One thing I should have mentioned: I just had my bike fitting last week and the bars were dropped down quite a bit. Combined with being new to the open road, and the new position on the bike, and new to shifting, my poor little thumbs are maxed out. I will try to vary my hand position and that will help -- I'm still one of those newbies who is always ready to brake. I did get the little extension thingy put in the brake levers to make them easier to reach, but my hands are super tiny. I guess I'll just have to get used to it by riding, riding, riding.