Betrayal is a moral injury, and sometimes it never heals.  

Understanding that being betrayed is NOT YOUR FAULT, is step one to recovering from it. 

Understanding that it is NOT YOUR FAULT that you didn't see the person for their true self...isn't your fault.  It's not a character flaw in you that you trusted them. It's a character flaw in THEM that they betrayed your trust. 

If you continue to suffer....you are giving THEM the power to hurt you.  Fuck them.  Don't let some asshole continue to be a source of pain.  Let that shit go.  They do NOT DESERVE your friendship. Remember fondly all of the good times you had with them.  Be grateful for the good stuff.  Grieve the death of the relationship.  Grieve the death of the "good person" who you thought they were.  

Don't let others keep you down.  Fuck them. 

Be healthy, be happy.  Find new friends...better friends. Move on.