Some people find it essential to take frequent breaks. And, like the U2 song goes, some days are better than others. Try to bring a mental list of some alternative things you can do if the anal part of your sex just isn't working. Often, just knowing that you have options is enough to relax and allow pleasurable anal penetration. A good partner will be patient and allow you to adjust gradually to the fullness.
Patience is important for both partners.
Most of us are taught to feel shame regarding our buttholes. True, the anus and the rectum are the passageway for feces. Yet feces are not normally stored in the rectum except when the body is preparing for a bowel movement, and a healthy person eating enough fiber should have fairly firm stools that do not linger. But in any case, it's a bad idea to get fucked on a full stomach. This can definitely cause cramping and other problems.