What helps me is to keep telling myself that what is being triggered is a bunch of bad feelings from a long time ago. I know I have to work on these feelings, but I don't have to accept that they're true. I try to tell myself that the past cannot hurt me; it's over and gone.
That doesn't mean I can turn off the trigger like turning out the lights, but it does help me to ride the thing through to the end without reacting in a way that will harm me or make me feel bad about myself now, in the present.
Hope this helps.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)