I have encountered the same phenomenon, i have found it easier to wear a diaper and use it towards sexual satisfatiction with computer images, solo, versus an actual intimate encounter with my girlfriend. I might be different in the fact that my first girlfriend actually was turned off by it but was not sure how to tell me her problems. She ended up cheating on me and it degraded to the point of her and her new boyfriend laughing in my face about my desires. I find there are no negative consequences when nobody else has that much power over me. I do fantasize about trusting someone else with that part of my life, but have also found that it is hard to do. I have a wife who wants to be a part of it and seems to not mind, but my own fears have made it hard to be that open and vulnerable, which ended up destroying our intimacy, and ultimately ended in a separation. I feel for you, and at the same time feel the same way. If I knew the answer I would gladly share it. I can say with confidence that at least I have found an outlet, "Solo" activity is satisfying, if only temporarily, but it is enjoyable. I can fantasize about what I like and pretend in my mind during an actual "physical" encounter. The biggest obstacle is in my own mind due to the humiliation factor that I have to deal with myself. Good luck with it, sorry i couldn't give you an answer to your dilemna, just wanted to let you know your not alone.
Keith