Mark, the only way to describe this is to think of it like drinking alcohol, to get better at it you need to drink more till your body gets used to it. Flying is the same the more you fly and the more your exposed to it the more of a tolerence you build up. The RAF run a course for air sickness and all they do is make you almost sick then stop till you feel ok. Once you recover they start again until your not sick anymore.
Im afraid it is pretty unlikely you will manage to get over without spending lots of money.
I know its not first choice but there is always the civilian route to take.
Hope it works out.