"How could he do this to me?": You may never understand. And that's okay.

The one cloud that hung dark in the days/weeks/months after D-Day was this question: How could he do this to me?
Inherent in that question is, of course, a whole lot of fear. How could I have been so wrong about him? What does she have that I don't have? Is he lying to me now?
I was certain that if I could just understand what he was thinking, what motivated this behaviour, then I could anticipate it happening again, I could gauge the likelihood of a repeat performance, I could protect myself from further pain.
I've come a long way since then. I no longer torture myself at 3 a.m. with toxic worry. I no longer believe that I can choreograph others' actions. I catch myself when I begin to assume blame for others' bad behaviour. 
And though I've gained a lot of insight into my husband's choices, I'm not sure I will ever understand how he could do this to me. Not really. 
But I know something else. It doesn't really matter.
There are lots of reasons why people cheat. But most of them fall under the umbrella of "I liked how it felt." That can include the sex itself – the physical feeling – but more often it includes the psychological feeling. People who cheat like the excitement. They like the anticipation. They like feeling as though they've turned back the clock: they're sexy, they're interesting, they're young. And, frankly, who wouldn't like that? I can remember all those feelings from back in the day. I loved flirting. I loved knowing that the person I was with was dazzled by me. We're our best selves when we spent only bits of time with another person. It's easy to hide our flaws, easy to imagine that life will be easier.
Those of us who can't imagine cheating, however, have an ability to think a few steps further down the road. We can imagine coming home and looking at our partner. We can imagine the day he finds the text. We can imagine our affair partner giving us an ultimatum – him or me. We can imagine just how awful it must feel to betray someone who doesn't deserve it. Consequently, we can't imagine cheating. The price is simply too high. 
Cheaters? They don't get past the "I like how this feels" stage. Or, if they do, they go back and rewrite history to somehow justify what they're doing. We're nags, they tell themselves. We've lost interest in sex. We're probably miserable too. In fact, they wonder if we're cheating. Or if we want a divorce. In any case, if nobody finds out, nobody gets hurt, right? Seems like everybody cheats anyway. 
In my husband's case, cheating was like booze. It was a way of numbing himself from feelings he couldn't stand. Even before I entered his life, he relied on sex to to keep at bay his feelings of loneliness, inferiority, grief. 
Do I understand how he could do that? Not really. Not anymore than I can understand my mother pouring herself a vodka and coffee for breakfast.
But I can understand that a whole lot of people prefer distraction over feeling their feelings. It happens all the time whether I understand it or not.
So "how could he do this to me?" isn't the right question for me. It gets me nowhere.
The right question, for me and for any of us who want to rebuild our marriages, is this:
What is he doing to ensure he never cheats again?
Is he doing the hard work of figuring out how he did this? To understand the stories he was telling himself? To learn how he was affected by cultural messages, family messages? Is he willing to really feel his feelings – including those around his cheating? Willing to listen to your pain even though it makes him feel terrible? Willing to support you as you inch your way through days and weeks and months of trauma?
Because that matters far more than how he could do this in the first place. Sure it would be nice if these guys were incapable of cheating. But they are. And so what shapes our marriages from here is how far they're willing to go to repair the damage they've caused.
It's really really hard. The issues that led them to cheat are the same issues that make it hard for them to own up to it, to accept responsibility, to do the painful work of understanding why. I don't know of many who can do that alone, without the help of a therapist or a support group. They lack the emotional bandwidth, the psychological tools to heal themselves and, therefore, help you heal too. 
But that is for them to manage.
You? Your job is to stop asking "how could he do this to me?". You only need to know that people cheat because they're damaged in some way. Hurt people hurt people. Damaged people damage people.
They did not do this because there's something wrong with you. They cheated because there's something wrong with them. 
And they need to fix it.