The trauma of your story reminds me of how/when I found out my long-term marriage had been possibly destroyed by infidelity. It is devastating, truly. We all get it here whether we're newlyweds, boyfriends/girlfriends, young marrieds.
In reading all the comments here, a couple things come to mind:
DO make decisions (talking to a lawyer, getting STD testing done), but nothing long-term immediately. Of course you can have your WS come home if it feels right. Just don't make any promises you can't keep. As much as you don't want it to be, this may be a dealbreaker. Most likely, neither of you knows that right now.
DO go to counseling--just not to a marriage counselor first. MC comes after IC for both of you. In IC, he needs to figure out why he did what he did; you need to vent and cry and figure out some self-help strategies. Once you've both got things figured out a bit (time varies), then you can talk about how to fix some of the pre-A issues.
DO concentrate on yourself. If you've never been selfish before, now's the time. Join a gym, start attending a book group, learn to play piano, teach yourself Italian and plan a getaway vacation for your next birthday, buy a new outfit, get a makeover, invite a girlfriend to lunch weekly, spoil yourself with a mani/pedi. There's nothing special about that particular list EXCEPT that you're indulging yourself and REteaching your brain that you matter and that you are special and that you are worthwhile. That part of you has been crushed and needs some TLC.
DO talk about his A. He should be willing to answer ANY question you EVER ask him, multiple times. You can't unhear his answers, though, so be careful what you ask for. You should know her name, if she's a coworker, if she's married and has children, if she lives nearby. If she does, you should ask for a photo. She's probably seen a photo of you and you'd want to know if you were standing next to her at the post office, right? You might ask how long it lasted, how intense it was (EA for 6 months, PA for 2?), how often they talked/texted/called/e-mailed/met, what was the primary means of communication, how often were they intimate. I'd hold off, though, until you're more certain what you simply must know, on more details. If he's still "foggy," his answers could be painfully shaped by the fantasy of the A and, truthfully, be something you both regret talking about a few months from now.
My SLAWH, early on, while trying to be honest, said that his A was "good" for him. He meant that his self-esteem was stroked and he felt needed/wanted, and I understood that, but even now--15 months later--my brain still cringes whenever that particular thought pops up. And I know, now, that he realizes that his A was far from good. The ego kibbles were short-term; the shame/guilt/regret/remorse are lifelong companions.
Do NOT bring up the A every moment of every day. It'll come up quite organically--because of a news article, a friend's experience, a TV show/movie, a trigger for you. Try to walk that fine line between passion and grace. Let him see your anger and your devastation--you can't rugsweep that--but don't let him hear you say monstrous, unfair things about him. That's not playing fair. Even though this isn't a game, there are still some rules that have to be followed. Be your own referee.
DO allow yourself to feel hopeful as long as you temper that hope with reality. You shouldn't trust him now. He may be worthy of it in the future, but not now. Watch his actions, enjoy his companionship (if/when you get to that point), rebuild when you feel capable, communicate honestly, look for opportunities to create new memories and traditions, but stand up for yourself.
There ARE couples who've successfully reconciled. My SLAWH has too many additional layers of brokenness to be able to say with any certainty that we've made it. Or that we're even in R. But we're both making progress. I love him. He loves me. He's trying to heal himself and fix his issues while showing me how important and beloved I am to him. Considering how selfish addicts are, I'm satisfied with his progress. For now. But I'm worth more.