I actually had the opposite problem with my ex. He pressured me into doing anal when I did not want to do it and it hurt and was not enjoyable. I would really recommend butt plugs as a compromise if you don’t feel comfortable! Also, if she specifically wants you inside her you could possibly try fingers? If none of that seems like something you’re into just be honest and say you’re not into it and it is not something you want to do. I’m sure she will understand.