You've taught yourself how to squat on your heels. This can happen with someone that's been told they have persistent knee slide. Rather than get leaned over and send the hips back more, they stop the knees early and sit back on their heels. I say this for the benefit of everyone else reading this as an example of the pendulum effect we see when you try and fix one issue (incorrectly) and inadvertently cause another in the process. Not that I have definitive proof this is why you're on your heels.
At any rate, you can edge your stance in a bit so depth isn't as much of a struggle.
Let the knees come forward for the first 1/3-1/2 of the descent as you get your torso leaned over and send your ass back. Part of the reason you're losing back tightness is that you're not near balanced on midfoot through the rep, and your body is trying to figure out how to keep you from not falling one way or another.
Knees forward and out, ass back, drop your belly button in between your feet.