I think the first thing would be to assure yourself that talking about sex to your therapist isn't any worst than talking about the weather. Your therapist is there to help you and you need to be honest with them. Once you can bring yourself to tell her the truth, then you can work to get past triggering which sounds completely like a symptom of your PSTD.

Hang in there and talk with your therapist honestly. I'm sure it will help immensely.