(((SoVeryTired5))) So sorry. This must be very difficult.
I suggest that when you start to feel a trigger, think of the word "patience". Remind yourself that it is normal to have triggers given where you are at in R. Acknowledge the trigger by reminding yourself that the birthday celebration is your current reality and the trigger represents fears from the past. Tell yourself, I will deal with this trigger after the celebration is over.
The important thing is to not fight the trigger or try to bury it. Acknowledge it, understand the fears behind it, and remind yourself it is ok to deal with it at a later time.
Triggers are driven from the primal fear centers of our brain and can overwhelm us quickly. Fortunately, our higher brain functions can regain control over the fear and calm the response. That's why it's so important to think of the word patience, or some other word that causes you delay an emotional response. You then give the logical mind the time it needs to say everything is ok, there are no immediate dangers, turn off panic response.
Good luck and try to enjoy your WH's birthday. Let him know you are struggling with the fears of the past, but you will be doing your best to have a wonderful time.
Remember, loving feelings follow loving actions.