Fifi,
For the longest time, I also could not stop excessively focusing on my xN. I was feeling soooo bad and hurt for him, on top of the torrent of my own excruciating pain, caused by that very person. I was constantly analyzing his personality, and what went wrong in this life, crying over the pain I'd imagined he was in, hoping he would get better, still wondering if I could do anything to affect a change in him (geez!)
I realized that the reason I was still so invested, was that in the back of my mind, I always had hope for reconciliation. I had never let the “dream” go.
Currently, I think I have managed to accept that the things I had wanted with this person would never come into being, at about a 90% level. It has been really difficult, and it took a very long time after our relationship ended, but now, I am so much less focused on him.
What I wanted to mention as my point in writing, is that I think that when one lets go of the hope of change in the N partner, and lets go of the hope reconciliation, or “the dream” (of a life together, being a family again etc.), this is key. You become able to stop focusing so much on that person, and to put your attention onto yourself, and your life (in your case, your children’s as well).
If you can work to a point to truly be able to let go of any hopes of reconciliation or him changing (many times I’d convinced myself I’d let go but hadn’t—you really know when you have) you will just naturally, automatically stop thinking so much about that person. You won’t even have the desire to invest so much of your thinking and emotions in them. This is true, even if you still care about them, as I do my xN.
It may sound a strange thing to say, but I feel one could get over this kind of relationship easier, and stop all of the over-focusing, if the N partner had died (rather than relationship dissolution), because one would not keep holding the hope of change. There would be zero possibilities to "dream" about.
You would be amazed at how much you can change in your thinking and perceptions of things when you let go of "hope", related the N's. It places a much, much needed sense of finality around the relationship (relationships with N's are so often left "open", vague in standing, no closure) which will enable you to move on and focus on other things in your life.
Best...