A year really isn't that long, healing takes time, and different people heal at different rates.
have you tried some grounding techniques? I know they don't make the flashbacks go totally, but finding a few that work for you really can make them more manageable.
have you ever considered telling someone about this? even a friend or whatever? you don't have to go into detail, but having someone who knows even just a little can really help...
christmas is going to be awful because this person is a very close friend of the family.
I assume this means that you may be seeing this person over christmas? is there no way you could make an excuse to be out of the way when they visit or your family visit them? I can;t think of anything offhand... your profile doesn't say your age, but maybe you could use school/uni work as an excuse to keep out of the way? or seeing friends? just to limit the time you have to see this person
I really wish I could advice more... as danskpige said though, just take it one day or hour at a time. you CAN get through this