How can I release the anger and pain of being betrayed?

My BF has been hitting on my “BFF” for the entirity of our 9 years together. There have been a few occasions where she complied, didn’t resist, or was the one doing the flirting herself…mostly him, but still, she is not 100% innocent. I just found this out 2 weeks ago. I’ve cut them both off besides the fact that I live with my BF and we have a son together and he basically pays the bills. I will never be able to trust either again. I have so much sadness and pain inside me from all this…I am sick with it. How can I get over this betrayal and move on?