So, my boyfriend has been talking about anal sex since the beginning of our relationship. I've never said no to butt stuff, examples, we use a butt plug while having sex (one that doesn't bulge out like a normal butt plug, a three inch, inch wide one), he'll finger my ass while in doggy position and also, every time he eats me out, I come, but the second he puts a finger or the butt plug in my butt and fingers me, I have the hardest orgasm ever!! Anyways, I want to try anal sex, but I'm so scared it's going to hurt, and I've tried it before but I couldn't help but feel like I had to poop the whole time, even after relaxing my body completely. It also bothers me that my partner has had anal multiple times, and he just keeps saying how awesome it is, and I want to please him, but I can't get over my fear.

I know the typical use lube & relax, but how do I relax when he's moving inside me? That's when it feels the most uncomfortable. Also, what position?