Believe me when I say that I know the feeling.
I broke up with my ex fiancé about a month ago because she was an alcoholic, a drug addict, a cheater, and verbally abusive. As much as she hurt me, and even though she basically destroyed our relationship, I still miss her a lot, and I find myself wanting to go back. It's not as bad as it was, but it's getting better.
What helped me was writing out what I called an "angry letter." I already wrote her a letter explaining why I left, and I told her that I won't even talk to her until she's been in recovery, sober, and seeing a counselor for at least 90 days. In my "angry letter" I wrote out the deepest wounds that she gave me due to the drinking and the drugs. I wrote out how that behavior made me feel, and I voiced how angry that it made me that she did that. Whenever I find myself missing her, thinking about how much I loved her, and wishing that I could fix things, I read that letter to remind myself of how deeply she wounded me. When the thoughts of messaging her come, I remember how much she hurt me, and I remind myself that if I take her back, then I am asking her to hurt me again, possibly worse.
I've found myself missing her less and less after doing that.
I've also tried to shift my focus from the hurt from the past, and now I'm just working on a five year plan for my future. One that is much happier now that I don't have her causing chaos in my life.