Good job for taking lessons! And I can't stress how good it is that you are practicing switch while learning. I did the opposite so here I am into my 3rd real season snowboarding and I'm just learning switch, I can still hardly do it. Other than practice practice practice all I can say is keep at it and listen to your instructors.
For your stance angles I think 15/-15 is great, and 12/-12 isn't bad either. Keep both bindings at equal (but opposite) angles if you are trying a lot of switch. It might help you to keep your bindings in one place for a little while until you get used to riding, then experiment, but I'm sure others would disagree. Repitition is key to learning though.