Sadly, I don't know a spell that will help fear, I agree on trying a courage spell or maybe even a confidence spell.
What also works, is to make a list, a pro/con list write down all the pros about rollercoasters, then the cons. Pros out weigh the cons, if it's the height you are afraid of, then I understand that perfectly. Rollercoasters can be scary, but they also have an excitement to them. That's just my opinion.