My suggestion is just to force yourself or even before going to the park, tell someone to force you against your wishes onto the ride, lol.
When I was younger I was scared of coasters too, and didn't go on them. My family only visited a themepark once every few years so when I got back home afterwards I would really regret being scared and wished I had actually gone on the rides.
So the next time I went (which was probably about 5 or something years ago) I just forced myself to go on a Vekoma SLC (if only there were more reasonable first-time coasters in Australia at the time), and as scary as it was - I ABSOLUTELY LOVED IT! And here I am today, hanging around roller-coaster related websites, with annual passes to my two closest parks (Dreamworld and Warner Bros Movie World - yay Superman:Escape) and I go on everything and anything (that Australia has to offer that is, I still haven't managed to experience the rides in the States yet).
Even still, it was only at the beginning of this year when I got my annual pass to Dreamworld that I only went for the first time onto Wipeout, a Waikiki wave superflip. Previously I had been a little scared of the extended periods upside down and the erratic movements, and nowadays having forced myself onto it, it is among my favourite rides at the park - even moreso because of its good capacity so I can't ride it many times.
So my advice is - even if you are still scared, just force yourself to go on it anyway. The scared feeling is natural and will last just as long as the queue and loading does. Once you get on it, you will realise how much you love coasters and you'll then go on everything and anything in the future. Show your fear who is boss and don't let it prevent you from riding the best things in the whole world - rollercoasters!