Take A Breath
There is no need to walk around with a mountain of anger and resentment. Put it down. Let it all go, and relax.
Start with a blank slate.
This person, this EX. This is not your anything. This is just another person. This person means nothing to you. Can not hurt you, effect your life, or what ever they do matters to you.
They are no more than an ‘extra’ in your movie.
Once you can fully grasp the fact that whatever happened last year, or last decade is to stay there, you can move on. If you have to, dig a hole, and symbolically bury the relationship.