NIK is spot on.
My exwh is still with the slunt OW and for a long, long time I felt used up, thrown away, worthless. Every once in a while I still get a little niggle of that feeling somewhere deep inside so I can tell you that it takes a lot of time and reflection to let those feelings go. They are natural.
The good news is that what I can also tell you is that, while I didn't see this in the beginning, I see clearly now that it really is all about him. This had pretty much nothing to do with me or my worth or my looks or my ability in bed or what have you. Nothing.
For reasons that we don't need to go into right now, I never realized when we were in the marriage that my exwh was so weak and pathetic and scared of himself. I was too busy taking care of everything, working full time, and raising kids when they came along. I never noticed his true self and that's partly because he didn't want me to. He walked around with a mask on for a lot of years. Once it dropped, boy oh boy did I see a completely different person.
Running away takes virtually no effort. It's the path of least resistance after they realize what they've done. Once they see that they just detonated a bomb on their spouse and kids, they have to run or else something worse will be made to happen - they will have to face themselves and the tremendous pain they've caused. And they will have to stand up and fix it. That's a tall order, particularly for someone who is a natural born weakling in the first place.
My ex exudes weakness in everything he does these days. From the outside looking in, I can now see that he's a weak "partner" to the OW - he lets her completely run the show. He's a very weak father - he never sticks up for our kids or shows much interest in them when they aren't physically with him. He was a very weak son - his mother was ill for quite a long time. We lived far away from her - when he would visit, he would sit with her for a couple of hours and then go drinking with his best friend for the rest of the time there. He's a weak employee - he does only what needs to be done to get by. He's hated his job for many years and has never once made a true effort to find an alternative.
I would bet that your ex is a lot like mine. You just don't see it yet. You will. Once you get outside the bubble and you start to truly detach, you can look at him with a more objective eye. That's when you will notice that he was floundering with you because he was trying to keep up with a person with integrity and a strong core. They can only do that for so long - then the choice comes - to run away or to really make some tough internal changes that we will demand. Once again, it's easier to take off the mask and find someone who will settle for much less.
Read and re-read NIK's post. It's all about him.