I absolutely HATE roller coasters. Used to love them until I took my son on one and he freaked. Now he loves them, I hate them. Go figure.
Space Mountain is a concern for me because I've seen it with the lights on and know how high it is. I just go on it, and when it gets to an acceptible level, I have fun. Up to that point, I keep telling myself I will survive. Rock N Roller Coaster is fun for me because it shoots me off very fast. After that, I pull out every cuss word I know because no one can hear me over the music (well, maybe they can but I don't care). For Mission:Space, I just keep looking and listening to Gary Sinise. Don't care about much of anything else at that point. For Big Thunder, put your boyfriend at the left side of the car. You'll be smacking into him instead of the other way around. For Splash Mountain, you enter the log first. Then he'll get drenched after the big drop (which is cool but very short).
Twilight Zone always freaks me out. Not because of the drops but because it's such a mind game. If you keep that in your head, it'll be a breeze. Also, see if you can request sitting next to the sides. That helps me out since I can brace up against that. Also, tell your boyfriend there's no way in heck he's getting any handholds. I always hold on with both hands.
I'll be riding Expedition:Everest in two days. I'm telling myself I just have to do it. Just have to do it. I will survive. And with stories to tell.
Hope that helps. I figure I'm trying everything once and there's always a bathroom nearby.