Overheard husband declaring his love for another woman!

I’m not sure where to start. For 5 months I’ve been asking my OH if he’s seeing someone else. He would disappear without telling me where he was, always take calls in private, be distant and uncaring towards me and create arguments with me and so on. I suppose I wanted to believe he would never hurt me so I kept going. On Monday I went to see my husband after work as I know he is stressed and the office was locked. I heard his phone conversation with another person. I was floored. My soul shattered and I’m still broken. His conversation was a passionate argument. He was shouting ‘I love you, I just want us to be together and I won’t give up on us, I just want to be with you, I’m doing everything I can to show you I love you, the hot air balloon, the weekends…’ and that’s when I told him I heard everything. I walked off because I couldn’t run, I was shaking. It took him 40 min to contact me. He says it is just a friendship and that he confided in a ‘friend’ yet he won’t tell me who she is and is protecting her identity because she too is married. I’ve tried counselling the day after with him which was pointless. He made out that I was saying he was an emotional abuser. I told him yesterday that I am removing myself from the situation since he won’t give me the transparency I’m asking him for. I don’t know what to do. If I leave she’ll probably be back on the scene immediately. I know that if that happens then I need to move on but this has broken me. How dare he throw 15years of my loyalty, honesty, love and always having his back down the drain for a 4 month ‘emotional affair’?? I don’t know what I’m asking, I think I’m venting. I’ve lost everything