How to let go of someone before you lose yourself?
We are not guaranteed closure when relationships end. Often we become paralyzed by perceived injustices. Often the injustice of it can take one captive and leave one waiting for justice. I myself have experience with wishing an ex-boyfriend, "Would get his." You may be like me, where the guy likely has moved on with not a care in the world, but you are held hostage watching for revenge.
When you cannot confront someone who is deserving a dressing down, being able to express the feelings can be therapeutic. I often assign clients to write out their feelings so that those same feelings do not stay frozen in your psyche. Often unexpressed feelings will become fodder in our unguarded moments to change our mood with negative rumination. I do not recommend sending such writings to the individual as it will turn into focusing on a response, which can be equally frustrating.
Another option from the therapist toolbox is the classic empty chair technique. This option is often better for those who do not like to write. The empty chair can work with no adornment, but sometimes having a photograph, drawing, or a picture to attach to the chair will make a difference. The idea is to act as if the person deserving your wrath is in the chair and let your feelings go. The purpose is to vent your unresolved feelings, to the depth and breadth they exist. If you have a keepsake to represent the person in the chair, you can further your release by making a ceremony of its disposal. The idea here is to find a memorable place to dispose of it. One can bury it in the area of the yard where the cat likes to go, or put in the grossest bag of trash you have. Once when I was in therapy I destroyed my letter to my ex-boyfriend and threw it in a very nasty dumpster behind a restaurant. Being choosey about the disposal place will allow one to remember the resting place instead of getting caught up in the old emotionally driven thoughts, self-flagellation, and ruminating about hopes lost. It is time to move on! All is not lost! Vow this time to find someone who is a match to you. (Emphasis on "to" is intentional.)