It's not your job to find out the why; it's your husband's.
Is he in IC? That's really important. It would probably be helpful for you to see a (different) IC, as well. MC, IMO, is best approached a bit later, when you are certain that what you are seeing is remorse, and both of you have a few more tools in your coping toolbox from IC.
Sometimes, you'll see the advice given not to make a decision about your marriage for 6-12 months. I disagree with this; if you conclude, sooner, what you cannot live with your WH's behavior (now or in the past), or that infidelity is a dealbreaker, there is no sense in remaining even a moment longer.
But that's if you know---and you obviously haven't gotten there yet.
It took me several months to conclude that my husband not only did not have remorse or empathy for me (or anyone). At five weeks, I was uneasy, but he was saying the right words. It was over time I realized there were no actions behind the words. If I hadn't been as decimated by the infidelity, I would have seen this sooner---but I didn't. From the moment I discovered his affair on, I was very uneasy it would be continued or repeated--and, in fact, it went underground. there was a history. But I did not really KNOW until later. The details are not important--what was, for me, that there was an abrupt, unequivocal, "I can't live with this."
It just took time for me to learn that no, we couldn't repair things because a marriage cannot be repaired when one of the partners lacks insight, empathy, or the capacity for remorse.
In your shoes, I would take a deep breath, and just ....let things unfold. You don't have to make a decision today, or tomorrow, or even 6 months from now. You can approach your marriage as you would approach a new relationship (which, in a very real sense, it IS)---and assess whether this man, a man about whom you're learning difficult things, is a man with whom you wish to spend the rest of your life.
It's an enormous decision. It will likely hinge on how well he is able to address his issues and support you in your pain.
At five weeks out, you may have a remorseful husband on your hands. Or, you might have a man in the throes of the oh-shit-just-found-out scramble to make things better by "being good" and working to "get back to normal" (rug-sweep).
Time is a four-letter word around here. None of us likes hearing that time will tell, or help ease the pain. But it's true.
Take the time you need to determine whether you want to build a new marriage with this very flawed man.
(We're all flawed. But going into marriage we have expectations. You now know that he is able to breach one of those----and have to reconcile that with remaining with him. A decision to R may be eminently reasonable. Or it may not be. Only you will be able to make that decision.)