I had this exact same problem, my grip was "too strong" and my right hand was under the club more than over the top. He made me change it and bring it more over the top to a "weak grip" and forever more i was slicing much like you seem to be. I found when practising/playing i was over compensating and gripping far to much over the top (to the left), so switched to somewhere in the middle of the stong and weak grips to go to a neutral/weak grip.
Stick with it, it will eventually help you out much more and you'll start to get a feel for what your doing in the comming weeks and can ever so slightly tweek it yourself for better results