Hey there, HP.
From my own experience researching infidelity, there seems to be very little consensus on how it happens, why it happens or what types of infidelity exist. Studies focus on various aspect and miss others, draw unwarranted conclusions and make a great many false generalizations. It's an exercise in futility. The only whys and hows and causes you need to understand and the ones that pertain to your fiancé.
Like you, I was glued to SI whenever I had free-time. I read like crazy here, posted several times a day, often started multiple threads a day. It's a safe place. I quickly felt very welcomes and at home here. Nearly two years later, I'm still here, every day, though I don't post as much and rarely start threads. There's tremendous wisdom here and a wide, wide variety of experiences.
...putting names to the feelings I am experiencing, and knowing that they are not unique to me is very comforting and relieves my anxiety.
Yep! See my username? Brother, we all know the feeling. Infidelity is crazy-making shit! You're experiencing a trauma. It's real, it's deep, it's powerful. You're certainly not alone. Look at your member number.
Your life isn't going to be normal, or anything close to it, for... well, the foreseeable future. I still don't know how I managed to function like a relatively normal human being during those first few months (and I'm not so sure that I did, to be honest). It was a struggle to keep it together in social situations. After a while, I started to skip some of those special occasions. I didn't withdraw from the world, but I did realize that putting myself in awkward or uncomfortable situations just wasn't a very good idea. It gets better with time.
If you ask me, it's way too early for MC. Your fiancé needs to be in IC. You might want to go for yourself. I refused to go to MC. Marriage don't cheat. Marriages don't have affairs. Fixing the marriage doesn't fix the cheater. You might get lucky and find a good MC, but be aware that there plenty of stories here on SI about MCs doing more harm than good.
This leads me to my main revelation. She doesn't get it.
It takes some WS a long time to "get it." She will never "get it," however, unless she listens to you explain it. She has to be open to that experience, willing to truly listen, sympathize, empathize... feel that remorse. I think it took my FWW about 18 months before she really "got it" and I started to feel her remorse. It's up to you to decide how patient you're willing to be, how effectively you can communicate how you're feeling to her. The rest is up to her, and beyond your control. It's entirely possible that she may never "get it," which is why so many members here are encouraging you to prepare for that possibility.
Remember this, though. However remorseful she becomes, however far she goes to help you to heal, it's only help. You're going to have to heal yourself. It's sucks. It's not fair, or even close to it. But it's the simple truth. Healing is a choice you have to make for you.
She's going to make mistakes. You're going to make mistakes. Surviving infidelity and trying to rebuild your life without your WS or attempting to reconcile a relationship devastated by infidelity... either way, it's a steep learning curve and hard. Just remember to be gentle and kind with yourself. This shit is hard, brother. HARD.
I think I read "Understanding the 180" once or twice a week for the first few months. I kept a printed copy on my night stand. It's a good idea to implement it, particularly when she still works with the OM.
Also, Google "No More Mr. Nice Guy."
I just want this week to be a little better than the last one. That is the goal right?
It does get better with time, but you're at the start of this shit storm and it's going to last for quite a long time. Two to five years, hurtpenguin, is the average time it takes to recover, heal, and (possibly) reconcile. And the odds are stacked against you (not you personally, btw).
The most important thing you need to focus on is your own recovery and health. Eat, sleep, drink lots of water, exercise, etc, etc. Focus on you and putting yourself back together.