Anal hurts so badly!!!

I've tried anal a couple times with my bf, & I will admit it was uncomfortable but manageable. We haven't done that in about a year, but tonight we both wanted to try it again. We were doing it in the shower, than he grabbed the lube. When he put his thang in the a hole it hurt SO much! He did about 3 pumps but I couldn't handle it. It felt like a knife going up there! We finished it up with just regular sex but I feel so bad I wasn't able to give him that. He's not always in the mood for that, but when he is I want to please him in that way. He told me he doesn't want to hurt me & that we won't do it if it does. He was fine after, but I just feel so disappointed in myself. How can this happen after already 3 times doing it? He went in super slowly too...wasn't rough at all.