Help!!

So my fiance has been wanting oral sex really bad lately but I just can't get myself into the mood for it, I dont know why..well i do..when I was pregnant he came in my mouth and I damn near threw up and chocked alot (he likes when I gag and I dont) but I do try to make it his way but when I think about him finishing in my mouth I want to throw up. I would like to do it for valentine's day but I'm just scared? Any tips or tricks??