I get the thing about obsessing about what your H and the OW are "doing" as well as the random images that pop into your head. Like others have said, obsessing about this is an entirely normal response, especially early on. You will often think to yourself something similar to your last post: "I need to stop obsessing." Yet you will be inextricably drawn to let your mind go back to it.
However, there comes a point when you will realize that your obsessing is getting you nowhere but more hurt and less happy. Here's what I know: in terms of cognitive theory, you are what you choose to think about. The more you think about something, the bigger and more emotionally powerful it becomes in your head. (This is why overthinkers are far more likely to become depressed: because they're always thinking about the things that upset them!)
So any time you find yourself thinking about their "activities," make the active choice to think about something else. Read a book. Go for a walk. Think about a happy memory, or just be in the moment. At first, this "switching" trains of thought will be difficult, and you will probably have to do it consciously a LOT. After a while, though, your brain will begin to change as YOU change your patterns of thinking. You will begin to feel at peace more often. Thoughts about "them" will hurt less as time goes on.
I know what I'm talking about because I'm at this stage in healing myself. I promise, if you keep doing this, you'll feel LOADS better.
