Sorry to hear you are having problems, I think you will find some good suggestions from the others.
I found a good counselor helped me to learn my triggers (words, thoughts or where I am, etc) and then to learn how to control the situation. Like, the previous poster said use the word STOP. Say it to yourself. Yes, good idea. I have used that and other things to help myself. There are other things we can do, like some wear a rubberband around their wrist and snap it to get the thoughts cleared. Have a coin in your pocket and hold it for a reminder to not believe the negative thoughts. I have an angel coin
I keep with me to hold.
The best thing I can suggest is to find a good counselor. Someone to talk to and learn how to deal with your feelings. Everyone is different, and it can be best to find someone you can talk to privately. I went to therapy for years and finally remembered what happened to me as a child ( I lost 10 years of my childhood memories). Then the healing started, and slowly with working with my therapist I found ways to stop those messages in my head. The memories grew grey and stopped coming so much.
Your not alone, and if you cannot pay for therapy there are programs in the state to help you, free of charge or on a sliding fee scale if you work. Use them, that is why they are there. I was never denied help, and keep posting if you need help with anxiety or anything to do with PTSD.
Good luck, and you have a great friend. Cherish Him.