Dear Lara,
Just another POV here, a more simplistic one perhaps, and not as deeply psychological.
What was the purpose of this happening? Does there have to be a purpose?
It just seems so random
The purpose is up to you. You are the person who gets to decide if there was a higher purpose to meeting and being with this man, and you get to choose the lesson you learned, if you learned anything. You, not us, decide. Have you ever seen that essay on the internet about how some people come into our lives for a season, a reason, or a lifetime? Some people are only meant to be in our lives for a certain amount of time, and we don't always get to decide the length of time. Think of kids you knew in grade school that you no longer know. Think of people you knew because of jobs, that you no longer know. Think of people whose parents die young. Think of people who have a loved one die suddenly. Is there a purpose, a reason for any of that? I have a friend whose father was killed in a car accident. Did he ever feel there was a reason for it? I doubt it at the age of 14, but perhaps years and years later, he did.
Perhaps your exN was only meant to be in your life for the length of time that he was. That was all.
The other aspect of this which I strongly believe is that it is natural that you would still feel linked to a person you loved and felt deeply about. Of course you should! Even if he hurt you, and it ended badly, you did love him. I was with my exN almost the same amount of time as you (lived together 6 years, together 8 years) and eventhough he destroyed me at the end and devastated me, I feel terribly linked to him still. We had a connection, I do not think that is broken when you part after a considerable amount of time together and being in love.
Because we had financial business to finish after I moved, I said to my N, "You know, we are inextricably linked until we complete that transaction" and his response was "New Day, we are inextricably linked for always" .... I was actually surprised he said that, and it may have been a typical N maneuver, but I have to say I think it was genuine. And true. My exN moved on quickly, as did your's, Lara, and your's has a baby, and mine married the OW. But despite that, it does not negate the years we had with these people.
When you say you want to be done with all this, I feel the same, but what I really mean is that I want to be at peace with it. I do not expect the connection to break. I just want to be at peace. That is my goal.
Maybe that is your ultimate goal, too, dunno. But maybe give some thought to the idea that it's okay to still feel a connection to your exN, and that you will find a way to integrate it into the fabric of your life without discomfort eventually.
You get to decide.
Be well, Lara.
New Day