One person wronged me badly, and I was struggling with forgiveness. A thing that helped me was to list all the "red lines" they had crossed. It made it external to me, and any further rumination could be addressed with the question "is this already on the list?". When the day comes to explain why I am no longer a friend, I don't need to rant and rage. I can just calmly go through the list...

I still feel anger and resentment, and I am far from being at gratitude for the lessons I have learned, but it's a process made of lots of small steps. Betrayal is not an easy thing to recover from.