Here are six truths as to why I ended my A. Helpful to me whenever I get misty-eyed; hopefully helpful to people on this site:
Truth #1: He was going to be unavailable for a *VERY* long time. This meant an uncertain future. He has three (3) kids still in school; a co-parenting arrangement with shared rotating living arrangements w/ spouse; he was emotionally unavailable and was constantly texting/checking email when we were together, etc.... I remember hearing a text message come in on his phone at 3 AM. I wondered just who was contacting him at that hour!
When I was single/divorced 10+ yrs ago, I was completely free (I didn't have children with my 1st husband - and I still don't have kids into my 2nd marriage). I dated a man similar to my xAP - except that man was fully divorced. Even so, he was still very much entwined in the family thing and fought frequently with his ex. The children always came first - it was like a popularity contest with his wife sometimes... who did the kids love more, mommy or daddy. I tried hard to support them - fed them, took trips together, expressed love for them, etc... but it never felt right for me. It was like they were all in a survival boat together and I was in the attached dinghy behind them. So even after the divorce happens, it can take YEARS before someone can truly move on.
Truth #2: It was getting messy - I knew it, but he was always holding it together/holding it inside. He was going through a difficult separation and divorce (his wife cheated on him with a mutual friend) and he was keeping everything bottled up inside. He showed surprisingly little emotion and this was alarming to me.
They were forced to short-sell their home; he had to sort through their belongings, his tools, including big power tools he once used. He talked about it so factually with me, like it was just another activity. (If I were forced to examine, sort through and dispose of the evidence of my life, I would be terribly sad.) This was a red flag for me.
Truth #3: He was geographically unavailable. He lives nearly 500 miles away and was geographically unavailable. I think he wanted it that way - he could have it both ways - time with the kids/local friends & getting their sympathy that he was "alone"; when I was far away, out of sight and undetectable by people close to him.
Truth #4: The A wasn't enough for me and was not satisfying. The time that followed intimacy, in moving on to the day, was awkward and unsatisfying. I wanted him to hug me, look long and lovingly into my eyes; perhaps sneak a kiss or whisper in my ear. That just didn't happen. I don't think he knew what to do... it had apparently been so long since he had made love the first time we got together.
Also, while his text messages were beautiful, our phone conversations left me flat. In our last phone call, all we talked about was his new job, his kids, his this... his that. At some point, I wanted him to ask about me and how I was doing; how I was dealing with my own serious marital problems... I wanted him say how much he missed me; when he thought we could get together again; how he thought of me in the middle of his work day, etc... but he didn't. He was ready to hang up without repeating some of the beautiful things he expressed in his texts. It was unsatisfying.
Truth #5: I started to have feelings for him; I could see I was starting to get attached/needy in the face of an uncertain future. I never knew when we could be together. Plans were not made and I refused to make suggestions. I absolutely did not want to become attached to him (knowing all the the above truths!), but was heading there. I felt I was losing control and becoming a little needy. I found this to be the most difficult to deal with. I think when people get attached in a bad situation like this, and don't stop what's making them so, there's a good chance someone else will stop it for them. In my case, I felt that if I could not control where and when this A was going, I could at least have control over when and how it ended.
Truth #6: I was making him out to be more than he really was. He was kind of boring, really. His job was his life. He could talk up a storm about his (technical) work and his golf game, but beyond that, he struck me as being socially awkward (it's funny, there's a TV commercial running at the moment about a guy who gets dumped after his girlfriend tells him how boring he is - http://nyti.ms/OdwLa7).
I'm posting this so that I can go back and remind myself why my A had to end. I hope it helps some of the other posters to think about the reality of their own situation as well.