I'm in the same boat as you. I've been squatting for years but I've never really pushed my squat until the last 6 months. If I do manage to squat to parallel my back rounds, I bend forward, and the weight is on my toes. No matter how tight I keep my back, It's always a choice over staying upright, or hitting parallel I'm incapable of doing both.
I've been doing some extensive research (read: I googled it to death) and I'm convinced that in my case it's a hip mobility issue. I've got an appointment with a chiropractor/ART (active release technique) therapist on the 20th and I'm hoping that he can help me out.