Motion sickness is a complicated subject. Broadly people fall into four categories:
Those lucky B.....s who never feel ill.
The vast majority who feel a bit queasy when introduced to a new motion, car, boat or aeroplane but pretty quickly adapt and are ok thereafter.
A smaller group who adapt very slowly (in my case almost two years) and are then ok if maintaining a regular exposure to that particular motion. Sadly, in this case the adaptation seems to be quickly lost.
An unlucky few who will always feel ill.
One of the curious features is that individuals are sensitive to different motions. I've never been car sick but my wife who can stand aeros without effect, quite easily feels ill in a car. I don't mind bashing to windward in a yacht but felt ill very quickly on the rolling and pitching decks of Rescue Launches.
If it is of any interest, I've noticed over the years that far fewer people seem to feel ill in helicopters than fixed wing. I don't know if this is due to the different motion or the generally better view of the outside.
Whatever, always make sure that your instructor knows the moment you are not feeling so good. There is no point in trying to be a hero, you are just wasting valuable air time. I suffered from one instructor, an ex Black Arrows Formation pilot who was always taking the controls and honing his aerobatic skills. It wasn't a lot of fun and did nothing to reduce my sensitivity! Sitting there whilst someone else flies the aeroplane is much more likely to make you feel ill.