Hey 1LM, Welcome! You have come to the right place. I'm sorry that you feel so stuck and lost. Your story is quite familiar. I was married to my BPDxW for 16 years, with two kids, so I can relate to your situation. Be careful about allowing your pwBPD to slip rocks into your backpack:
He believes I am overreacting; that he thought the affair would be good for the marriage. He says this is my fault and has accused me of planning it. He has only recently been diagnosed as having BPD.
Those w/BPD are experts at shifting the blame to the Non, because it takes responsibility off their plate. Your task, I suggest, is to let the blame roll off you like water off a duck. I have a saying: "Poison is harmless if you don't ingest it."
The place to start, I suggest, is with yourself, by being kind and compassionate to yourself. Return the focus to yourself and your needs. What is the best path for you? You get the idea. Do you have any particular questions?
LuckyJim