I used to be an invertiphobiac, fear of going upside down. Its not as rediculous as fear of buttons but still pretty rediculous. Was so bad I couldn't even make myself do a handstand or anything. No idea why I was like that but yeah...
I never used to go on rollercoasters purley for that basis as I was scared of going upside down. Got taken on a trip to thorpe park and was forced onto all the rides (Collossus was my first ever roller coaster) and was white as a sheet of paper when I got off, but I didn't think twice about getting on the other coasters for the rest of the day.
If you're really struggling, get someone to force you onto one. Once you're strapped in, theres sod all you can do about it, you're just along for the ride.