Done anal years ago

So I have a steady fwb and he has a fetish for anal sex and wanting to lick me there. Last time I allowed it was 3 years ago and I felt very uneasy. Don’t get me wrong it felt amazing but I couldn’t fully enjoy it because I felt so much pressure I felt like I was going to poop. My biggest concern was pooping and just being embarrassed about it if I did happen which it didn’t but I would be if I did. What can I do to get over this fear and how do I prepare for anal sex. Since 3 years ago it was my first time and I didn’t prepare myself. It just slipped in my wrong hole and I stood quiet. He didn’t notice until like halfway through sex.