Time was a part of it.
Working with a counselor that is strongly based in mindfullness. I took a meditation course which helped me to see that thoughts were not permanent and that helped. Realizing that others in relationships like mine have experienced this too helped me to not feel so alone (like today).
For awhile I had to write lists of what I was mad at and take them into my support (my counselor mainly). I think I am trying to say I had to get them out first, then in the light of day (and words) they became easier to deal with and lost power.
My ex wrote a poem years ago that I found in the midst of our worst. It was sad, and it was about how alone he felt in the world. I held onto that for months into our split to show what a stand up person, and lost soul he was. I did not share it though and knowing he still felt that way made me feel horrible (which was ALL mine).
I brought it to my counselor and read it to her. She said I hope you get rid or that (and I did), but sharing it with her helped to take the power away that it held previously.
I know I have gotten better because my mind does not race at 100mph ALL the time. When it does race those are the pieces I have not been able to let go of yet. I have some tools in place to help now with that, but mainly I try to get more support when it happens.