I'm 24, and I have ADD and coasterphobia. My girlfriend is a fanatic who loves all the rides. I haven't been on a roller coaster in 12 years, and the last time, I got off a ride dizzy and unable to see or walk right. I now go along on park trips for a nice day out—but end up wishing I'd stayed home. I sit there hating myself when I have to see people thrilled by my terror. She expects me to be excited for her after she gets off a ride; that's asking me to ignore my feelings. I want to do rides and enjoy life like normal people.
I hate to break it to you, but there is plenty of excitement outside of amusement parks, and it is perfectly possible to live a full life without ever going on another roller-coaster ride. Roller coasters are designed to turn fear into fun. Obviously, it's not your brand of fun; that is nothing to be ashamed of. It's inconsiderate of your girlfriend to insist on going someplace that feels like torture for you—and it's self-abuse that you agree to go along. Your needs and wishes deserve as much consideration as hers in planning how to spend time together. Your social life is something you should be jointly negotiating. Start speaking up for your own needs in this (or any) relationship—and respect yourself enough to believe they deserve equal play. Your girlfriend can go to amusement parks with others. You need to be spending time together doing things that satisfy you both. That she ignores and even enjoys your discomfort suggests you're in the wrong fun house altogether. If you feel she has redeeming virtues, then at the very least, don't collaborate in her willingness to unnerve you; next time she wants to go for scary rides, tell her you prefer to seek amusement elsewhere on your own.
Advice: Ups and Downs of Fun (Psychology Today)