oh hun, christmas stuff IS hard
is there a chance you will see abuser? or is it more that being with that group of people triggers fears/memories?
I also really understand your frustration. I used to feel forced into celebrating christmas when I didn't believe in it and actually hated it. It was incredibly hard and i had to be very internally strong, but i stopped celebrating it. told people i did not want gifts and refused them when they tried to push gifts on to me. told people i would only go to eg lunch for immediate family and wouldnt go to a large gathering. that kind of thing. it took many christmas's for my choices to be accepted by others. but has been a huge step forward in my growth.
but i know that is scary and hard and may not be what you want. smaller things you could do may be to refuse to be somewhere that abuser is. just go to small gathering, not a huge one. just commit to going for a short period (eg brunch rather than full dinner). have your own way of leaving so you aren't dependent on waiting for others to want to leave. take a talisman that will help you (i had t's hanky, but it may be something that soothes or something that makes you feel strong). allow yourself to hide in bathroom or somewhere private if you need some space or need to take a little time to soothe/ground self. give yourself permission to call bf if things become very difficult. you can also give your own meaning to the day (eg make it a day to do something beautiful for self and do that thing before you go away to do the whole family thing. eg might be something like time in nature, put flowers in bedroom, give self time to write, read a book that talks to inner you, also other comforting things or soothing things). you can make it a special day that has nothing to do with christmas.
you can get through this hun. just takes careful planning beforehand and a committment to helping self. be gentle with you - especially if your anxiety levels increase closer to the time

and kudos to you for posting hun - allowing yourself to be vulnerable is couragous