It might also be hamstrings. For example, while sitting on the chair try lifting one of your legs so you make L shape with your torso (without flexing your lower back).

I can't do that, my leg can't stretch fully. This prevents me from squatting properly.

I've been slowly stretching the hamstrings but I'm still not there. I remember having the same issues when I was a teenager.

It is very unlikely that hamstring is preventing you from squatting properly. Hamstring is nowhere near the end of it's range of motion in squat because it crosses both knee and hip joint and knee is bent during the squat - therefore hamstring is not considerably stretched.

Ankle flexibility is more frequent cause of not being able to squat properly. Hips also, but not because of hamstring.