Try the stretch that Mark links to in this article: https://www.marksdailyapple.com/squa...p-glutathione/
I had the same problem as the reader who asked the question--I would fall over backwards when trying to do that squat, but after doing that stretch for a while, I'm now able to squat. Mark and the SF CrossFit guy in the video call it a calf stetch, but I think it's really an ankle stretch. I did destroy two pairs of running shoes doing the stretch though. But it was worth it, and I still try to do it most days of the week.
Just keep working that movement. You'll get there. There is a degree of balance involved too so like some have suggested us a pole or door to hold onto as you come down into the position till you can get there.