The relevant choice is not between BS or AP. The relevant choice is between mental health and not mental health. Choosing AP = not mental health.
I was in this same boat. I thought my AP was a fantastic person. I came very close to leaving BS for AP. I decided to stay with BS for 6 months and figure my shit out. I felt I owed him at least that. (I am so grateful to BS for not just kicking me out while I insisted on sitting on the fence.)
What decided it for me? Well, d-day was in June and the day before Thanksgiving I was in town picking up vittles for the following day and my six month deadline was fast approaching to make a decision about sending that NC letter. There had been no contact between AP and me, but I had still refused to write and send and NC letter. Still on the fence. In fact at that time I wanted to go back to cake eating, could not understand why BS was not willing to share me with AP. I knew it was unconventional but...I was just a special case, you know, in love with two people, all that.
So there I was at my beautiful thinking spot and there was an old man there enjoying the same beautiful spot. Staring out over the ocean we started talking. Both of us had at one time lived close to this spot. His daughter still did. He was visiting her for Thanksgiving. He and his ex-wife had an arrangement as so many divorced couples do, spending every other holiday with their kids.
Somehow he started talking about the reason for his divorce. "She wanted to be married and have a boyfriend too, and I was just not okay with that", he said. "Hmmm," I thought, that sounds like me. They had been divorced now for years. I asked him if he remarried. He said no, but he was well over the divorce and dating and happy with his life. "How did you get past it?", I asked. He said God got him through it, and it took several years.
I asked, "What about your ex-wife, is she still with the boyfriend?". "Oh, no" he said, "She is three or four relationships down the road from that now. I don't think she is ever going to be happy."
To this day, I believe that man was a messenger. I wish I could say that I was thinking at all about BS's happiness right then, but I was not. I was only thinking about my own. At the very least, though, I got a clear view of my possible future if I chose AP: Three or four relationships down the road and still unhappy.
I went home and wrote a NC letter. It took me several tries to get one that BS was okay with but my willingness to do it was born in that moment when I realized that the choice was not BS or AP. It is mental health or not mental health. That day I got on the mental health path and I am so, so, so, so, SO glad I did.
That man will never know the impact he had on me. I thank the Universe for sending him to me.
As far as your AP is concerned, no matter how fantastic they seem, remember that your AP is someone who is okay with you being a liar. In fact your relationship with them requires lying each and every day. Your relationship requires you to get your needs met at your BS's very great expense. Your AP is fine with you doing that too. Does lying and getting your needs met at other people's expense, people you purport to cherish, does that sound mentally healthy to you?
It's not. Choose mental health. Do it for future you, if for no one else.
Good luck from a fellow EvolvingSoul.