Sammie, I was in the exact same situation as you. Married for 20 years (together 28), two teenagers and separated due to him having multiple affairs. He too is now with his affair partner, she was also married.
I wont lie, I did go through a tough time mentally, I found the divorce difficult as it was the finality of it I think, but we were separated longer than you before divorce proceedings, I think a quicker divorce is better as it doesn't drag it out as much.
I am 3 years on from there now, and things are a lot better. During the 3 years I had quite a few woe is me moments, sometimes I just lay on the sofa for the entire weekend if the dc were at their dad (he doesn't see them now but that is a whole other issue) and binge watched series. Looking back i was a complete slob during those times, but I needed to do it. I don't regret it as i was just me time, non thinking time, as I do tend to overthink massively.
There are so many benefits. I am less stressed and so are the dc. We have formed such a good bond now, as its just the 3 of us in it together. We are open and discuss how are are, although my eldest ds does battle to get his thoughts out so when it happens its like a volcano erupting, but its ok. Its out then and we discuss whatever we need to, have a plan of action and do it.
With regards to how to treat ex. We tried to remain amicable through out it all, but truthfully, it is very difficult. Bearing in mind he has ow on his side and her wants and needs are being taking into consideration, whereas my whole focus is on the dc. So at this point in time we only speak if we have to, I would rather have it that way as i get upset as he still comes out with packs of lies.
I haven't mentioned all the benefits of getting out of an unhappy marriage, but there are so many. In myself, I am a better person. I am happier. I look happier. I get compliments all the time about how good I am looking now. If i look back at photos of during the marriage, it so difficult to see, I just looked so sad ALL THE TIME. Its so obvious now but I was flogging a dead horse and it was doing me no good. (he had affairs previously as we tried to make it work, it didn't). I changed what we ate, as I wasn't just eating what everyone else wanted to, I lost weight and that alone gave me so much more confidence.
I did have a very strong mindset that I would never move on to a new relationship, I really felt that I couldn't go through all of that again and felt i no longer had it in me to give what a relationship needs. And I was happy with that decision. Someone came along not that long ago that wasn't expected, and i think i am getting a little more used to the idea of moving on it that area as well now, but I am just taking a slow, just day to day as it's clear I am suffering from some issues due to the breakup.
It still early days for you and there is a process you will be going through, upset, anger etc. You have to go through it though, to heal. But there are many of us in and been in your situation and for those who are coming through the other side, I find the majority as so much better off mentally.