If you give yourself a thorough enema before anal play, you can enjoy the sensations without worrying about shit coming out- that may free you up to relax and enjoy the experience. Go very slowly, start small, and work your way up. You have to be completely relaxed- even just a little tenseness will hurt really badly, but if you relax, it can be very pleasurable.

The first time you have butt sex, the "have to shit" feeling will be noticeable, but if you are relaxed and he is gentle, the sensations become good, rather than bad.
-Phantome