Can anyone offer any advice on how to turn off my constant thinking about this?
I had a real problem with rumination. At over ten years out now, I rarely think about my WH's affairs, and when I do, the hurt seems far away albeit remembered.
I can't tell you what to do, but I can share some of the things that worked for me:
1. I realized that rumination was a separate problem than my WH's cheating. It had to do with my brain, and the way it was stuck on the problem, like a vinyl record going round and round with the needle digging a deeper and deeper furrow. Through research, I came to believe that rumination initially has purpose in the healing process, but past a certain nebulous point, it becomes a bigger problem than anything which happened in the past... because it's happening NOW. It's messing up our present because it's in the way of current goals.
2. I had to turn my focus to my goals, meaning that I needed to jump down off the fence. I think it's normal for the newly betrayed partner to become indecisive and that that particular state of being goes on for a long, long time, and certainly past the point where we're doing variations of "fake it 'til you make it". Initially, I had determined that I would TRY for reconciliation. For me, that meant giving it an HONEST effort. In the early days, I literally had to check myself to see if I was behaving in a way that was true to my promise to try. But later on, those efforts had become habit, and as we know, there comes a certain comfort in habit and time marches on. Meanwhile, my ambivalence had never truly resolved. I was still just "trying". I needed to jump down off the fence, one way or the other and OWN my choice.
3. Owning my choice means that I am where I have decided to be. No one (and nothing) has MADE me stay. I recognize that I have all the facts that I needed regarding the past to make this decision and that I'm the one who has determined what ground I'm going to stand on. I can remake that choice in the future if I feel that circumstances have changed, but for now (and with acceptance of all the facts), I CHOOSE the ground I'm standing on. I choose to be here. I choose this partner, as he is today, and not as I might have wished him to be.
This was probably the most important piece for me... "owning my choice". Because it gives me back my autonomy. I am not a victim of past circumstances. I have put the past behind me and am making a choice for my present. I recognize my own POWER to make that choice. I know what happened and I am choosing to move forward simply by taking full ownership of the choice I make today. While it's true that anyone could have made me a victim, only I can decide to stay that way. When I seize my own autonomy, when I recognize that I have all the power over my present and my future, I leave victimization behind.
4. I had to reevaluate some of my own dogma. For instance, something I noticed we had in common... the word "soulmate", the belief that my fWH was somehow destined to be "my" person, that he somehow completed me, as two halves make a whole. The more I examined these bits of story that I had somehow adopted, the less they made sense to me. Moreover, I recognized that they were actively increasing my suffering. There's no real evidence to suggest that I might not have been happy with another mate, or even with no mate at all. There's no evidence to suggest that I am not a complete and whole person unto myself. In fact, the more I invested in my own sense of self, the better I felt, the less codependent I became. Today, I'm bulletproof in terms of continuing on in my marriage. It doesn't mean I wouldn't feel sad if we ended it, but I wouldn't be destroyed the way I was when I discovered the betrayal.
5. I don't have the science for you, but I believe that there's a little leftover bit of reflexive behavior that we carry from infancy to adulthood. You know, babies are born with a fear of abandonment. It's reflex. It's why babies cry when they can't see their mothers. We're born helpless after all.
I read a copy of The Journey from Abandonment to Healing by Susan Anderson, and it really got me to thinking about this. In fact, in all the things I read, it made the most sense of how our brains deal with abandonment issues. Intimate betrayal IS an abandonment wound, right? And here we are, born with the fear of being abandoned. It only makes sense that this fear may have been lurking all this time, and our need for connection, transferred onto the nearest and dearest person in our lives, right?
Here's the help though... we aren't infants anymore. We're adults who are complete and whole unto ourselves. Yikes! What a revelation that was for me. I don't NEED my relationship anymore. I might WANT it, but the loss of it is no longer an existential threat to me. That breaking, that severing of dependence, was devastating at the time. But my panic, when reliving it through rumination, turned out to be remembered hurt. I had been forced to stand on my own two feet without the connection I thought I needed... and I had survived. Turned out, I didn't "need" it after all. I'm an adult, and I can trust myself to be there for myself. Today, my connection is with ME. My relationship is gravy. I am meat and potatoes. 
6. Still, there's a bit of contradiction. As I recognize my power and invest in my sense of self, I also divest myself of suffering through EGO. It's an almost inexplicable distinction, but I will try....
Buddhists tell us that suffering stems from ego... basically, our attachment to our own dogma. We adopt certain ideas as part of our identity, what we believe to be true of ourselves. But when we challenge all that, we can end up freeing ourselves in many ways. For example, when I really sat with it, a lot of my suffering involved the thought "How could he do this to ME?" Turns out that my "ME" was written large, capitalized and in a humongous font. lol
In reality, my fWH is his own separate person. He's not an extension of me. He makes his own decisions, for his own reasons. He should have been thinking of me. But he wasn't. And when he was, his thoughts were colored through his own lens of irrational discontent, his own ego.
I felt like this shouldn't have happened to ME. But when I shrunk my "me" down to reality-size, I realized how inflated it had become by my own hurt. My hurt was so huge it called into question everything I thought I knew.. love, God, connection, human existence, you name it. The plus side is that I had some of those things wrong. I'm just a regular-sized person, and even though I choose to be my own "special person", I'm not so special that bad things can't happen to me just like they do to anyone else. I had no reason to believe that I should somehow be immune to disappointment, abandonment, or even betrayal. The realization though is that those things are about other people, not about me.
Anyway, I hope some of that made sense. Healing is a long process. I do think that there's value to therapy, particularly trauma therapies like EMDR (eye movement desensitization and reprocessing). For me, EMDR took the heat out of my worst triggers. Again, I don't have the science for you, but I believe that in trauma, our brains can fail to store our memories correctly, leaving us feeling like the trauma is still present rather than past. EMDR helps us to sort it.
For me, I needed to find ways to put rumination behind me. The more you work through it, the more you begin to realize that it's similar to that old parable about the Two Wolves...
"An old Cherokee is teaching his grandson about life. "A fight is going on inside me," he said to the boy.
"It is a terrible fight and it is between two wolves. One is evil – he is anger, envy, sorrow, regret, greed, arrogance, self-pity, guilt, resentment, inferiority, lies, false pride, superiority, and ego." He continued, "The other is good – he is joy, peace, love, hope, serenity, humility, kindness, benevolence, empathy, generosity, truth, compassion, and faith. The same fight is going on inside you – and inside every other person, too."
The grandson thought about it for a minute and then asked his grandfather, "Which wolf will win?"
The old Cherokee simply replied, "The one you feed."
When you find a way to feed your new wolf, the old wolf fades away. Our brains are basically organic computers that we program ourselves. The neural pathway of rumination is the default pathway, the familiar one. It's pain and suffering are what we've become accustomed to and that pathway is the LENS through which we view everything. Getting off that pathway requires that we GROW a new wolf from virtually scratch... so NOT easy. But totally worth it, and completely doable.
*please forgive any typos. I've run out of time for proofreading*
