Alright this is an issue I've had since I started working out.
I'm sure you've heard it before-
I can't do squats.
The details- I can do the squats in perfect form, but none once I get to a certain point, one of two things happen, either
-my heels lift up
-I fall backwards.
Again this is while doing it in good form. The only way I can avoid both is by bending my back forward which as you know is bad news.
I also noticed that my left achilles tendon is quite a bit tighter than my right, I figure it may have something to do with that, as I have a feeling that my right leg should be able to bend the correct way to allow a full squat.
I don't know if the tendons can, over time, be stretched and therefore extended, orrrrr if I'm just straight up S.O.L.
(You don't need to read this next part, its just a bit of what I've gone through searching for am answer)
I've looked to body building forums and it tends to end with muscleheads and their "aggressive motivation" putting the people who ask down. This is the first time I ask myself. I've gone to gyms and they've tried to help me but none have ever been able to give me anything that helps. Nothing beyond the terrible "heels on a weight" advice that I heard/read can screw you up in the long term.
Google Box squats. That might help you out.
Also... Not doing traditional squats is an option. Because my gym has no squat racks and I just never felt right in the Smith Machine, I've mostly switched to doing dumbbell split squats and I feel it's much better for my health overall and I feel the burn in my legs even more now.
Maybe try spreading your feet a little father apart seeing or maybe the weight needs to be moved slightly up or down where your holding it on your shoulders /back
With your heels coming off the ground you might find it helpful to keep your whole foot flat on the floor but just lift your toes up and this should help. I think it would be helpful for you to squat with perfect form for a week or two even if this means using lighter weights this way your brain gets used to performing the movement and it becomes natural to use the right form. You might also try do isolation exercises for the glutes to strengthen the muscles that can stop you from falling back.
To help with your tight achillies just sit in a deep squat position (w/o weight) with your whole foot on the floor. If you cant do this just sit as low as you can until you gain the flexibility to do so. Just sit in this position for time whenever you can and hopefully this will help.
I have the same problem. I have been doing goblet squats which I'm told helps train your body to complete a back squat with proper form. I also had to improvise because the gym I attend only has smith machines. Squatting on it feels so unnatural.
I am always quabbling with my squat form, comparing it to others and beating myself up for not being 'perfect' or for not lifting as heavy as i see others.
Im constantly reading stuff and asking people loadsa questions, these are a couple of things ive come across that seem to put limits on peoples squats...
1) Tight hip flexors
2) Ankle dorsiflexion
I wont go into too much info but utube has many vids on how to stretch these. Theres a particularly good one for ankles..https://youtu.be/XISJxsccN1E..
I found that the more i squatted the better i got. I could still improve a whole lot more but with some regular stretching and practice ive become so much better then when i started. I also noticed that many people i watch squat dont have this 'perfect' form that everyone harpers on about but they are squatting, are getting results and are not injuring themselves. Hope this helps!