Help! How do I sort this out?!?!

My husband says that his best friend is a girl he used to have sex with years ago. I have asked this girl to block him because he started being flirty. And she did. After that he complained for days how I ruined his life and took away his best friend. He said I now made him miserable.

I have talked to her. I’m trying to get to know her. But still deep down it’s killing me. I hate this. I feel like I should be his best friend. I don’t see how someone who would never date him but have sex with him is someone to be friends with. She refused to date him them.

I honestly feel like he loves her. Me and him fight all the time recently. And now he keeps saying if my attitude doesn’t change he’s going to leave because he can’t take anymore. (This girl has been around our entire relationship so I don’t think this part has to do with her)

Now that I have talked to her and she and him are talking again he’s being all sweet and wanting to talk. I have no desire to talk to him. I’m numb. My heart is broke. He doesn’t understand how I feel. All he cares about his himself.

And now I just found out he told her about our 2 miscarriages. He won’t even talk to me about them 😢

Don’t judge just opinions please