Oh, I see that your neck and shoulders hurt from the Serotta. Missed that before! I'm also seeing that you bought the bike from another rider, not your LBS. Still, my advice is the same. Test ride more bikes.

Maybe some of the other women will be able to address this-- theoretically, wouldn't a too small frame put less pressure on necks and shoulders? I would think that you'd be more upright, with less weight on your hands and arms, more on your butt. Or, would the neck and shoulders be a problem just because of scrunching up?