Re:squatting through the heels?
Unless you have some physiological issue (which I doubt if you're saying you can hit the heels on lower weights) then you just really have to concentrate.
CK is right about keeping the bar lower, be sure as well that you're bare footed. You just need to concentrate on the heel drive and focus the power.
Deadelifting properly will also help with ths as it's easier to get the power through the heels on a deadlift, and the skill is transferrable.
When you start the drive, start from the heel. Through the lift the worse you'll probably get is neutral, so at the end of the drive out think about the heel again - most people pop forward at the top of the lift, so concentrate your efforts there. If you start well and finish well, the form will hold through the mid lift.