Boyfriend’s fantasy

So to start from the beginning, we have been together for 8 months....now I know nearly everyone has fantasies about what they would like to experiment but me and my boyfriend got talking one night ( must of been at least 3 months in) and I was telling him my fantasy and then he told me his now I know some peoples fantasy can be extreme but this just shocked me as I didn’t think he would like something like that anyway I told him I would think about and see what happens.

On Valentine’s night I decided to perform his fantasy on him for him (even though I was not too keen on it in the first place) but I did just because I knew it’s what he wanted to try! Anyway from that day on he just wants me to do it more and more and I’ve told him how I don’t feel comfortable doing and how tbh it makes me feel sick but I know it keeps him happy.

He wants to do it again very soon and I just don’t want to/think I can do it and I know that’s going to upset him and I know I’m not going to be able to please him! I love him lots but I just don’t know what to do because I don’t want our sex life to be damaged over something stupid like that!

Anyone have any advice or has anyone else been in this situation?