Boyfriend of 3 years suddenly has female best friend.

My partner (m36) and I (f35) are approaching 3 years together in a committed monogamous relationship. Not married/no kids. About 1 year ago, his then co-worker who he had no relationship with, nor even liked very much, suddenly became a "best friend." We had arguments here and there about it, but I didn't believe there is anything more to it other than he wanted more friends. However; over the last 6 months or so, our relationship has really gone downhill. From my perspective, we do not have a strong emotionally connected relationship at this point. The sex has been good up until recently, but I feel that is because it has turned more into just sex and less like making love. Anytime I bring up things that have hurt my feelings or seem amiss, he tells me I am being negative and complaining. I feel like I can't talk to him about hardly anything anymore, good or bad, as he is generally too tired to listen. I'm feeling like he is either putting all his energy ino her or being drained by her. I found out that he has been talking to her about our relationship, along with her own issues within her life. He said he was "getting a girls perspective on arguments we have had." I told him I did not feel comfortable with him bringing others into our relationship issues, since we (her and I) were "friends," too. Not to mention, her personal relationship history is a trainwreck and I am not sure her advice is actual helpful advice. I learned that he talks to her for hours in the middle of the night while I am sleeping, or when he is out for walks and I am not around. They text every day. If we fight, he immediately texts or calls her in private. I feel like they are emotionally cheating on me at this point. I told him how I was feeling and that I am extremely hurt by this. His response was that I am being ridiculous. I told him that even though her and I have always gotten along, I don't feel comfortable hanging out with her anymore, nor do I want the 2 of them spending time together without me, which they do somewhat regularly and often I find out after the fact. I know I would feel much differently about this had they been friends prior to our relationship, but that isn't the case here. I KNOW she would not be OK if it were her boyfriend that was suddenly this way with another girl. She has been cheated on, and also been a cheater in the past. It may not be related, but it seems like most of our issues started when they became bffs and started sharing intimate information. Prior to their close friendship, him and I were great and were talking about marriage and our future together, which at this point isn't even a discussion topic. It is starting to feel like there is more going on between them than they are saying and it is affecting our relationship and my mental health. It feels like he is choosing her over our relationship/me. I'm not sure what to do or how to feel...