I've never actually barfed but I certainly came close to it while doing spin training in gliders when the instructor was demonstrating, that horrible 'Am I or aren't I going to' with the little sickly hiccups that you get. As soon as I did it for myself I felt fine. I also felt definitely fragile towards the end of my endurance flight for my silver C, but I think that was more lack of hydration, poling a glider around in anything but straight and level flight for 5+ hours and the chocolate I took to keep the blood sugar up. I can't abide chocolate. I think that once you start taking control of the aircraft you will feel a lot better. I hate being a 'passenger'.
One of the guys at our gliding club was an ex Buccaneer back seater, he used to barf virtually every time he flew in Buccs. At least he got paid for it.
Stick with it, I doubt that there's ever been anyone who has aviated who hasn't gone green around the gills at some time.