Why? Emotional succubus for my uBPDbf

[Marvis]

I'm not sure if anyone else's BPD does this or let's this happen but maybe someone can chime in theories as to why this can happen.  Here goes... .

A few months back my UBPDbf admitted to an emotional affair with a girl (in no way shape or form can I call her a lady but those are my issues) he is now her boss. She knows how to get to him, piss him off, full well knowing he can and usually will take it out on me when he gets home. She thrives on this.  She has it out for me, she's holding a grudge against me for him choosing to be with me instead of her(yeah, pretty sure she's BPD as well) He came home from work today all kinds of angry and finally said to me in a very confrontational and mean tone that he is going to text her to "solve their issue". I very calmly asked him what happened. He had no answer and started screaming at me, literally screaming, face red, mouth foaming. I calmly said "I am only trying to understand what the situation is so maybe I can help." Continued screaming non-sensical things. I let him finish and said nothing. I didn't try to explain or even mention that I felt that he isn't being truthful with me. It isn't about me. I let him "calm down" and went in his hidey-hole and told him I love him and again said I was just trying to understand the situation and if you need my help I'm here. He said nothing.  I'm now the enemy. My question is this, why does he let the bad people, who he full well knows will suck all the good, loving, kind parts of his personality straight out of him, get to him like this? He knows she does this on purpose to "get back" at me, she has told him this. Why does he feed in to it? Why does he want to fix "their issues" so badly but the things between him and I are MY responsibility to fix? I left the house just to get some air and I cried all the way to my jogging spot. It hurts to even think that he may value whatever the hell their relationship is more than he values ours. Issues with them come up, he tries to solve them right away, issues with us, he ignores me for 5 days. Perhaps I'm reading in to it too much but this is how I feel at the present moment. In his world,  I'm the b***h who is controlling him even though I never once said you can't do that. In his world I attacked him for asking about the situation. I was calm the whole time. He escalated the situation himself.  It always tends to happen this way. I'm confused, shocked, amazed, a bit hurt but most of all I'm NOT angry even though he thinks I am. Any input gladly accepted.

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[Marvis]

Forgot to mention that he has completely shut down due to this situation with her. He does this EVERY time. I'm sure I'll be attacked later for "not caring about him" "you're a burden to me" etc. Come to find out that is what happened last week when he left me naked and confused in bed after us talking through an issue then told me to leave him alone. She called him while we were fooling around. His phone was in the same room. He must have seen it to know she was calling. 5 minutes after that he got up and left.