I may be wrong here, but I think that letting go is tied very much so to your "ideals" of co-parenting...
I thought that co-parenting was a given..always. Right up until there was a situation with my Daughter, and saw that what my consequences were, played no part in what she lived with with her Mother.
A friend suggested that I google "parallel parenting" and read up on that.
What I realized was, that in my trying to "co-parent" that my ex felt that as some form of me trying to control her actions. And that she probably felt like I was trying to run her life , even away from me. At least that is probably how SHE felt.
Truth is, she was probably right. I still expected things to be as they were, and they weren't. I was working toward an old common goal , and she was working toward being a single parent.
When I started working toward being a single parent, was after I read about parallel parenting.
That is when I dropped the last rope tying our daily interactions together.