On the teacups, you might focus on the teacup, or the other people who are in it, who appear stationary to you. But everything around you is spinning, and your ears tell you you are spinning. The brain becomes confused. Sometimes it is better to focus on the outside. Others can't handle that, and its better to focus on what is still. Some are more sensitive to what direction the teacup itself is spinning.
I think I keep changing my focus spot, thus leading to a far more "exciting" ride. Yes, I do stumble a bit when I get off. But I am ready to go back on again.
None of the WDW rides give me a problem as they are rather "tame" compared to other amusement park rides. You know, the "big" ones. Not sure if I would handle the big rides now as well as I did twenty years ago when I was last at Great Adventure in New Jersey.
I do hate to admit that Space Moutain shook me about a little more than my body was happy with. I'd still go on it again, but at some point, I'd have to listen to my back instead of doing what I want to do!