I'm trying to end the affair but keep going back and forth between my lover and spouse. How do I choose?
It is normal to have conflicted thoughts & feelings during this process. When you are confused about your choices, it usually means you have conflicting goals. Real change is hard but nearly impossible if your motivation for change is unclear or conflicted.
Let me tell you a couple of motivations that WON'T work in the long run:
Guilt. Trying to make things "right" just because you feel guilt/shame or because you're trying to do what others say you should do will work for a while, but probably not for very long. Sooner or later, your own needs/desires will demand attention, and you'll be repeating the same choices again.
Happiness. Most people I work with are just trying to find a way to feel happier. They want a relationship with more "connection," better sex, or with less conflict. We all want to be happy! The problem is that any new relationship feels good at the beginning. (And when you add in the secrecy that is part of an affair, the thrill of the experience is heightened even more.) But it doesn't last. What thrills a person today won't thrill them a year or two from now... and they'll have to start looking for the next relationship that makes them feel that way again.
Both of these motivations usually rely on external measures (outside of you): an outward standard of correct behavior or relationships/circumstances to make you happy. In my experience, clients motivated by a desire to avoid guilt or find happiness often go back and forth in their choices. Healthy, consistent change is driven by an INWARD motivation when people have a clear picture of who they want to be.
You need to take the focus off of deciding what person is more exciting or gives you better sex or whatever. I'm not saying those things aren't important, but they are secondary to a more primary question: What kind of person do I want to be? If your answer to that question is clear, your choices become less conflicted because you begin to see which ones help you (and which ones hurt you) become that person.
In my opinion, answering the questions "Why do I think/act the way I do?" and "What kind of person do I want to be?" needs to be a primary focus for someone coming out of an affair. It's a process that requires you to look at your past (the things that influenced how you experience people and events), evaluate your motivations, and develop a clearer perspective of what future change will look like.