Sounds like a similar problem. I can barley keep up straight while going into a squat.  Like you said torso gets thrown forward.  I can barley manage 30 seconds in a couch stretch with the left leg back.  With the right leg back its no problem.  And the falling back sounds exactly the same because without weight to balance myself it just roll back. Feels like I cant get my pelvis forward or my ankle to bend more to balance.  Ill have to work on the hips and calves like crazy.  I have done some, which I thought was enough but it sounds like you have experience with this. Thanks Ali