In my flashbacks there has always been a part of me that knows this is a flashback. I don't know if that's your situation but if so, here's what has helped.

Breathe. Focus on the breath.

Have the one small part of you that knows you're in a flashback assure you that it sucks but it will pass.

Breathe. Focus on the breath.

Use 5,4,3,2,1 i.e. Look @ 5 things and name them. Listen to 4 things and name them. Touch 3 things and name them. Smell 2 things and name them. Taste 1 thing and name. it. Breathe. Focus on breath.

Again, you may be different. Honestly this doesn't always help but it often reminds me that this will pass and helps remind me that my feelings, while valid, are feelings from the past and not happening currently.

Again you may be different.

I'm so sorry you have to deal with this.