question about how to say no to a certain thing during sex

so I have a question. there's this one thing that my bf likes me to do during sex that I absolutely hate. I've done it maybe 4-5 times within 2 years with him and he really loves it. tbh I keep telling him I dont like it, but he's always like "do it for my bday/christmas/whatever"

how do I be assertive with him and be like "no I don't like it"?? I really hate it and would let him do that with another girl bcus I don't want it. how do o tell him so he understands? bcus for some reason he won't take no and like guilts me into doing it.🙄😔

please judge me kindly. thank you❤

btw our communication is the best it's ever been in our 5 year relationship, we try to work a lot of things out, this is just sensitive for me.