I had a similar problem. Can you do a pistol squat? I couldn’t. I learned that I needed alot more adductor activation and use to squat properly. Once my external rotation/activation was sufficient, I found that I needed to use more strength in my adductors to keep it all tight. Stretching my calves behind the knee to the heel (soleus, not gastroc) and the anterior hip was huge forme also. My calf kept pulling my heel up and limited my hips and then my tight anterior hip wouldn’t allow my pelvis to go into neutral when I was low (i had really bad butt wink) in order to rebalance or I’d just fall backwards. after lots of couch stretches, I can squat without falling back and I notice my torso doesn’t get “thrown” forward but can stay upright.