What you are describing is a clear tendency of a high wing trainer with dihedral. There isn't a problem with your airplane if this assumption is correct. I would'nt make any control surface mechanical adjustments because this can cause a reverse reaction when flying upright. Just learn to apply forward stick when inverted and make sure you have plenty of power. Many trainers don't fly well inverted for reasons already stated. I find that most trainers require all the power you can get for inverted flight.

DBCherry is right when he says that if you can fly a trainer and proficiently perform aerobatic maneuvers then you are about ready for an upgrade to your 2nd. airplane. You will love the difference. Good luck........