The reason why you lean forward (in your case too much) is probably because of ankle inflexiblity (ie tight calves). When you descend down into your squat, your butt sticks out. To counterbalance that, your body shifts your weight forward (knees traveling forward). If you're ankles aren't flexible enough your knees can't travel forward to counterbalance your butt. Regaurdless, the body isn't going to let itself fall back on its ass and look like a fool, so it shifts weight forward to keep you balanced the only way it can-- by having you lean forward (placing stress on your back).
The key to solving this is to work on increasing your ankle flexiblity, which is usually just an issue of tight calves. Loosen those bad boys up and your knees will be able to travel forward more easily, which will allow your body to balance itself, which will keep you from having to lean forward so much just to stay on your feet.
Good luck with the training bro.