Have you discussed this with your instructor(s)? If not, do so. They have probably met people with similar problems before and helped them overcome it.
You might also want to see if motion sickness medication helps - remember that you need to take it well in advance of flying. Ask the pharmacist for advice on which one to try, bearing in mind that it mustn't make you sleepy. (sorry no personal experience of this kind of drugs as I only suffer when exhausted and of course I don't fly then)
Also don't fly on an empty stomach, or on a very full one, and make sure you keep well hydrated and comfortably warm.
Someone at my club relied on ginger biscuits at the weekend and the end result was messy. Maybe it was you!
Finally, the sensations in the front of a 2-seat glider are different to being in a single-seat, motorglider or GA plane as you are sat a long way in front of the CoG etc., and when the glider turns or pushes over you get a lot more movement than the chap in the back seat, or in a single-seater. You go solo in the front seat, so that's where you sit to learn, but I guess it's possible a few back-seat rides might help - I suspect that you are reaching the point where you are getting anxious which is making it worse. (It's also easier to fly co-ordinated from the back seat!)