I have always been afraid of coasters that went upside down, but I bit the bullet and rode the Rock n Roller Coaster on the first day (gulp) of our honeymoon. I was so upset that DH scheduled MGM the first day and that we were supposed to follow his guidebook and begin with the Tower of Terror and then the Aerosmith coaster. Since I was so scared of going upside-down, I didn't want my "will I or won't I chicken out" moment so early in the trip. Illogical, right? But so is my fear of going upside-down (I guess?).
Anyhow, I was terrified in the line and practically hyperventilating once I was locked in. But, I followed DH's advice to put my head back, and I also improvised by putting my cheek to one side of the bar and closing my eyes! In the end, it was scary but really fun. DH thought it was hilarious that I, the screamer, did not make a peep. I couldn't really tell that we were going upside-down, but I never believed other people when they said that.
In a way, it is less scary than Space Mountain (though way faster) because you don't have the sensation of height (at least I didn't). Also, I almost prefer the launch to the slow chugging up the first hill that many rides have. That scares me more.
I also felt pretty safe with the bar over my shoulders.
I think you'll be fine! I won't EVER ride a coaster that is high up and outside, but I plan to ride the Rock n Roller coaster many more times in the future.
Good luck!