Roller coaster's aren't scary. Grow some chest hair and ride one.
Been there too, it was awesome
Carowinds Intimidator four times straight, in the rain for two. Nighthawk twice within an hour.
Anyway, just go on it or tell your girlfriend you dont want to go on it and if she tries to force you tell her to forget off
Top Thrill Dragster is fun as hell. You should seriously give it a go. Millennium Force is pretty cool if you aren't down for dragster.
http://www.youtube.com/watch?v=zGK8ezytHNsSeriously though, rollercoasters are perfectly safe, only fear is that some can make you black out.