How to get past this?

Start slooooow. He has to put a generous amount of time and work into getting you relaxed enough to the point that it doesn't hurt.

You should already be turned on. Then he should start playing with your butt. Like maybe some rimming and finger play. Just start with a well lubed finger. Some people recommend latex gloves for finger play. They say it actually feels better with the glove. We never use one. Once you feel comfortable with one finger, he can try two. I don't know how big your BFs penis is but if he is average then you will likely get there, eventually.

You could start with a toy, like a plug, that has a smaller girth than his penis. But again, you need to warm up before you even try to insert the toy. This is part of our foreplay... my husband plays with my ass and then inserts the plug for me. Then we might have vaginal sex with the anal plug in and then sometimes, we even go watch TV with the plug still inside me. When he recovers enough to have sex again, we go for the anal.

Another thing I can recommend (because it was recommended to me and it worked) is to start in the spooning position...ya know, where he cuddles up behind you as you both lay on your sides. After all your anal foreplay, lube up really, really good, relax and let him SLOWLY try to enter.

I still consider myself an anal novice but I just stumbled on a technique recently. While he is inserting his penis, bear down...kind of like you're going to the bathroom. This actually opens up your anus and makes insertion easier.

Once you finally get to the point where he's inside you, maybe just lay there a bit, not moving. Get used to the feeling. Then when you're ready, he can thrust in and out VERY SLOWLY. You're not gonna be getting slammed like in the pornos. Not in the beginning. Hell, I'm not even sure I want to have anal sex like that.

For me, the slower he thrusts and the longer his stroke, the better it feels.

If I had to sum it up, I'd just say relax and go slow.

eta:
My first time was a nightmare. He didn't even get his penis in and it scarred me to the point we didn't try again for many years later.

Also, in the beginning, I really didn't like the sensation. To me it felt like I had to use the bathroom. And it can still feel like that sometimes, if we don't go slow enough and I'm not totally, 100% into it. But once everything is right... I can tell you... it can and does feel incredible.

But don't feel bad if you never enjoy it. Not everyone does. It doesn't mean there's anything wrong with you if you don't like it. And of course, never let anyone do anything to you that you don't like.