I think BF is very wrong in saying it is not about you. It is. It is about your health and also about your relationship with his daughter(s), which apparently is almost non-existent. Tell him this; that you will go if he will go to your ex-husband's at Christmas. (if there is an ex)...put it into perspective for him.

I've had the same surgery and let me tell you, it takes almost a year to get back to normal "inside." Stress is not your friend either when one is trying to heal. Although this is his child, you are his life partner, yes? Your health should be parramount to him. If it isn't, then I would re-think HIM.

It seems crazy to me to ask your boss for more time off, endangering your job (livelihood) to go to a function you don't want to go to in the first place, to be with a bunch of people who don't want you there.

Another two cents worth.