When I was gliding I always found it much easier when I was in controll rather than having something demonstrated by the instructor. Fortunately I haven't had too many problems when flying but when i was sailing you could guarantee that if I had to go down in to the cabin and spend 10 minutes at the chart table with my head down plotting my position and I would be ready to vomit. Getting back in to the fresh air and looking out towards the horizon would cure it pretty quickly. Taking the helm, even quicker.
Eating crystalized ginger helped me but there is some debate as to if this is a medical effect or a placebo. The pressure point wrist bands available from Boots (other chemists are availabe!) also worked. So, my advice would be to tell the instructor at the very first sign of nausia. If you can, take the controlls and fly straight and level whilst looking out at the horizon. Try to avoid any sudden or jerky movements of the aircraft or your head. Try to get some fresh air on your face. If there is somewhere safe to store a sports bottle of water then try taking a sip of water to clear the "taste" from your mouth.
Keep trying. It will probably get better. On the boat i was more likely to be sick at the start of the season. By the end of the summer when i had been sailing regularly I had a cast iron stomach.
Finally, I tried not to take stugeron as it left me drowsy. Not great when you are in charge of a yacht doing 6 knots..... definately not good in an aircraft!
Good luck!