Not that I am a great squatter (755@242, 18 y/o) but use a box as a teaching tool. That is how I teach people.
I scare them into sitting back. I make them do it, and pull the box back, and tell them if they don't push their hips back to where they need to be, then they will miss the box and fall.
Of course I wouldn't let that happen, but they don't know that. I suggest you sit the box far enough back that if you do not sit back enough, you will miss it, then just instinct will tell you to sit back to the box.
Also wall squats are a great tool. Furthermore, give up the crutches of holding onto stuff or using the smith machine. If you keep doing it, then it will make it harder to learn how to squat the right way, IMO.
The front squats may be a good idea, but not really sure about that. It has helped some people.Generally I do not sit BACK as far on front squats, so I don't know if it will help that issue, but it will definitely make it to where you will dump if you get on your toes... so it may be another tool.
Elitefts.com is your best source for info.