When I started I too felt rough in bumpy conditions (or even when not very bumpy) for about the first 15 hours (often ended up going home and lying down afterwards!). After that time, the motion sickness went away.
When I did my IMC (i.e. not looking out at all), felt a bit rough initially, but again this reduced so feel OK in IMC even in very rough conditions.
It seems also that all instructors seem pretty immune, so my guess is that this is something that you get used to and the ill feeling reduces.