Tips on anal sex!

Me and my boyfriend wants to try anal sex. We try to but I squirm around a lot because I get this feeling when he is inside I need to go poop. I’ll get all anxiety about it and tell him to stop. I know we have to have lube. It’s just the anxiety of him going in the hole where stuff comes out and getting poop on his penis. I also read bad experiences with anal sex and it scared me.