U2 - I used to suffer dreadfully from airsickness, which stemmed primarily from my w****r of a flying instructor on the UAS who used to derive great pleasure from trying to make me throw up. It nearly put me off flying for good. However, I stuck in there and subsequently became a flying instructor myself. During this time I had to learn to teach aeros and spinning, not something I was looking forward to. Fortunately I stumbled upon an excellent instructor who listened to all my worries. He taught me slowly within sight of our home airfield (in case I felt unwell) and reminded me that lots of people get airsick and that it was very easy to make someone else sick whilst you were flying! So my advice would be to fly with someone who sympathises with your predicament and shout as soon as you feel unwell (no heroics required). Try and fly theses manouevres as much yourself. You will probably find you have more tolerance than you thought and find that your feelings of airsickness are more a fear of feeling airsick and letting yourself down.
Good luck
SD