Also don't feel pressured to let go. I rarely raise my hands on rides, just because for me its a safety net. I was scared for years and years to go on roller coasters, I'm not really sure how I got over it, except one year, I went on a roller coaster, that was just hills, no loops. And after that I wasn't scared. I would say try a ride that looks like "hey I could maybe do this" one that scares you, but doesn't seem to bad. Screamin might be a little bit much for getting over the fear.
Also realize your stomach will clench up. Mine always does, and that was one thing that scared me too. I hated that feeling of being nervous. Now the only ride I feel like that with is Tower or Terror...and I don't know if that will ever change no matter how much I ride it. Another thing that helped me too was to scream. So scream your head off. It will let the pressure out of your stomach and help relieve some of your stress. Good luck, and I know you can do it!