you said the usual trigger is your father and your soon to be husband /or alter (I wasnt clear on which one you meant) talk about you and the trauma you have gone though...

that tells me the solution is talking with your father....

I dont understand why your father and your fiance are talking about the trauma you have gone through.... usually a persons father and fiance dont talk about traumas that the daughter or son has gone through... usually parents and fiances talk about things like welcoming him to the family, and other normal conversation like sports, jobs, common ground things...

example when my sister got ingaged the discussion was about past family outings, quirky things my sister did like getting stuck in the washing machine when her friend dared her to climb in to get the lone sock stuck on the bottom. talk about the dysfunctional side of my sister like her past alcohol drug use, sexual abuse, and treatment never entered into the conversations. the bottom line is that fiances usually talk about traumas between each other not between another family member and the fiance... unless you are the one that brings up what happens to you...and in that case since you know the trigger is discussing your traumatic history when your father and fiance are together, is a trigger then again the solution is simple..

pick other times for you and your fiance to discuss you traumatic history, not when your father is around. and if your fiance is still getting triggered by it you may need to find yourself someone else like a therapist that you can discuss your traumatic history with, so that you are not triggering your fiance.