Oh, btw, I wouldn't call yourself a "troll" if you mostly just read theads. A troll is actually a negative term used for those who are annoying on purpose and try to incite arguments and attention on themselves.
If you have trouble playing through a fantasy, I gues you have to ask yourself.......
1) maybe the fantasy is not for me? Sometimes a fantasy sounds a lot better in your head than it does in real life. You might not know if it's a fantasy you truly like unless you try to play it out. If you can't go through with it, it could your body and mind telling you that you're not actually into it.
2) If you know you really like the fantasy and you're just shy or awkward in playing them out, I would recommend letting the provider know in advance and ask her to try and push through even if it seems like you are pulling away or not really liking the experience. Roleplay can get awkward with someone you just met for the first time. If they know you might not be as responsive as normal, that might help things out a lot.
I have done just a couple roleplays and I had a similar reaction as you but the provider never gave up and pushed through my awkwardness like a champ and I had a great time

But there are some fantasies that might be left untouched. Like I've always had this fantasy of bringing a provider with me to an asian massage parlor and sort of combining those two worlds by having the provider join in on the massage pretending we're a couple or married. Maybe one day.....