I'm thinking that either...
A.) You have a really weak back. Do you ever do deadlifts?
B.) You're using too much weight. The sticking point in suqats is right around parallel, so partial squats (above parallel) are much easier.
C.) You're using poor form. Try not to think of it so much as squatting down as sitting in a chair. Stick your butt out - as long as you're leaning forward a little bit the weight should keep you from falling over backwards.
"I don't want them to like me, because when I'm in the ring, punches fly to the face. It is not a tenderness time. I am not falling in love. And he knows that when the fight starts I am kicking faces."
- Wanderlei Silva