I still grieve the lost of my marriage.
Over a decade later I still am very sad that I had to not allow my exhusband back into my life.
Nothing or no one has filled that void. I have done everything to keep him, myself & my adult son connected. Mainly through coming together for my son's birthday drink. It is only just us when we come together. Unfortunately it does not seem to have the same significance to them.
Families should always cherish each other because divorce for myself has come with the greatest grief, loss of my own family home, family times, extended relatives & friends.
I was disrespected & for standing up for myself I lost the most.
Grief effects us all differently and I have to say that in each if my 3 break ups with durations 7, 11 and 10 years long, I never really moved on until I found the next lady I fell in love with.
My current wife had our 10 year anniversary last year, I've been the happiest out of all 4 relationships.
In your situation meeting up with your ex with enjoying your child would certainly stir the pot of "regret and what could have been".
Following one breakup I considered going back to her. She was the mother of both our daughters. A friend said "never go backwards as you return to the same fate". So my ex remarried and eventually her husband split from her. He rang me and met up and- he explained he left her for exactly the same reasons eg abuse, manipulation, narcissism etc.
So while regret is hovering it isn't productive. If you still desire a partner don't give up nor settle for less than the best.
Happiness is complex, clouded by the past but the horizon should be your focus.