I think you put your feelings into words EXTREMELY well, and this is no easy subject!!! I have great difficulty reaching orgasms too. I don't want to be anyone's little project. Also, I shut down if I feel the pressure is on, to hurry up, come on and orgasm (thoughts in MY HEAD ONLY, but still very present and enough to shut me down). I am very sensitive to the efforts of my partner. I don't want to overextend him. Sometimes stubborn works, but once I suspect he is not enjoying it, (EVEN if he IS enjoying it but I have a slight doubt), I'm done! I simply cannot see my orgasm as a gift to someone else, as a reward for their efforts. TOO MUCH PRESSURE.
Lately I have been trying to enjoy the other lovely things about sex, and not letting the orgasm be my main focus. I have a BOB* that I can use at my own leisure to take care of the physical release. Didn't someone say that my orgasms are my own responsiblity? I think I read that somewhere, and it was freeing, for me. I love people too much to put that expectation on them. Especially when an orgasm only lasts... how long? I mean, we're talking seconds!
However, I do feel that when there is a third party involved, it is hard not to be very aware and sensitive of what is going on with the other two. Comparisons can really get you into some trouble. Personally I think it would benefit you immensely to have some one-on-one time with each of your lovers. It sounds like you might be able to relax a little bit easier.
*BOB = Battery Operated Boyfriend