Not sure where to start, or end. WAW?

[TimeHeals]

Gah, I'm starting to feel like this is ALL my fault.

It's not all your fault.

I could have paid her more attention and been a better father and H to her before D-day and she never would have had this EA.

She didn't have an EA because of anything you did. People have EA's because they either lack poor personal boundaries (e.g. sharing intimacy with members of the opposite sex other than your spouse) or because they lack integrity.

You don't control her actions, so you aren't responsible for her affair. Let it go.

My life has completely turned upside down,

Things have just changed is all. Fighting the idea that things have changed causes suffering. Accept it, feel what you need to feel, formulate a new plan of action and stick to your core priciples and values.

I just really hate myself right now for allowing this to happen

You didn't allow it. Your wife is a woman with free will and can make her own choices.