Digging

I have a very simple problem, however, the solution is less obvious. I am developing a bad habit of digging under the ball and leaving a big divot - sometimes bearly touching the ball, because the divit is so deep, other times hitting a 7 iron 80 yrds, when it should go 140yrds because the club slows from taking a divot just before imapct. I have gotten some lessons - which identified lowering my back in the backswing as the main cause of this, however, the instructor has just said "try to keep your head up" - no drills, no specific way to work myself out of the situation.

On practice swings everything works perfectly - I skim the grass - just as I want - however, when I do my normal "the ball is now involved" swing, things seem to revert back to my bad habit.

To be honest this is all that is keeping me from enjoying a simple round of golf now. My 3 wood from the tee and my chipping/putting are at a standard that I am content with - I'm only playing a few months. But, I really see no hope in sight for getting around this digging habit.

Does anyone have any advice?
Thanks.