I too have a big fear of coasters, but I've pinpointed it down to the rides height/drops. I literally get scared waiting in line for rides THAT I'VE ALREADY BEEN ON AND ENJOYED.
I went to KI a week ago with the plan of riding Diamondback last but when we got there my friends wanted to ride it first because of the long lines it usually has. So I psyched myself into thinking "well the ONLY thing scary on this ride is the first drop, and it's now or never". We all got on, got off in one piece, and then had a chuckle of the picture of me holding onto both the seat and the restraint for dear life. The ride wasn't too bad and it set me up to ride every other roller coaster that I had previously turned down 2 years before with the same guys. The neat thing is that I wanted to ride every coaster again after finally conquering them.
My 3 step guide for conquering coaster fear:
1. Ride Backlot Stunt Coaster / Adventure Express (which I had liked 2 years before)
2. Ride the biggest ride to set you up saying "These rides aren't as big as the one I already survived."
3. Close your eyes while going up the lift hills (I do this on every ride until I'm at the top, somehow it helps)
Also remember that nobody ever dies on coasters unless they have health problems; they are designed to make you feel like you are going to die but you're gonna love it in the end.