Glad to hear you're doing as well as can be expected. The very best thing you can do is see if you and W can sit down at the kitchen table and roughly agree on the components of your divorce. If you can't, then let the lawyers handle it.
The components are:
1) The custody agreement: What percentage of the time will each of you have the kids, what will the schedule be, how will you handle transitions, and how will you handle holidays and school breaks. You need to agree on all of that. I had a friend where their agreement specified that on the handover day, one parent dropped off at school, and the other parent picked up from school. Then, there was a snow day with no school, and they got into a big fight about who had to do the driving between the two houses. The more specific you can make the plan the better.
2) The financial agreement: Child support will be a state formula based on your income difference and your custody percentage. These are after tax dollars for the person paying child support. The second component is alimony -- in MA the entitlement is 80% of the length of the marriage up to 20 years, and after 20 years its for life. Alimony can be negotiated, waived/etc. You should agree on what you're going to do here. Alimony payments are pre-tax, so if you pay her, it's pretax dollars and she has to pay the tax on it, so that makes it preferable to giving her a lump sum up-front which would be post-tax. Finally, there's the division of your assets and debt. This is where people can get really hung up arguing over the value of things. i.e. you're going to keep the couch, she thinks it's worth $4,000 and you think it's worth $200. Try not to get sucked into that. If both of you can "give" a little more than you're comfortable with you both win.
In my case, W and I hashed this out between the two of us and just hired a lawyer to draft it into a format the court would accept and my total legal bill was about $2,500, hers was less.
Regarding confronting OM, that's up to you. Generally, she's going to resent you for doing it and it will push them closer together but you need to decide if you care.
My exW had a few EA's and one PA. The EA I caught her in, I did confront OM and he was very apologetic and promised to go "no contact" and lived up to his word. For me, it felt really good to confront him and gave me a bit of closure.
The next time around, three years later with the PA, I did not confront OM. OM's wife had already found out via a private investigator so I figured there was really nothing to be gained.
In some cases, OM is a "predator" like someone mentioned above. More often, it's just some shlub that got caught up in it. In many cases these things are the result of a thousand micro-escalations instead of a decision that someone has made. i.e. two people are unhappy in their marriages and start discussing it with each other. The validation feels good, after a few get togethers one of them says something only mildly flirtatious. The other person likes the attention and doesn't object, or says something mildly flirtatious back. The next time it's slightly more flirtatious, etc. etc. and before they know it they're way over the line.
At that point, they are not thinking about you, or their wife, or kids, or anything else -- they're just enjoying the attention. They are of course aware that they're doing something wrong, but they're ignoring it and putting on blinders.
If you confront it will tend to shake them out of their fantasy world, but their reaction to that is totally unpredictable, they may deny, they may apologize, they may flee, or they may fight. In any case, it makes things with W worse. For me, that was worth it the first time but its a personal decision.
Acc
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015