Ugh, heartache is literally painful. I'm sorry you are going through that.
For me, getting iver someone has been a multi-pronged approach.
Accept that I'm going to feel grief for a while...at least a few months. That's okay and normal. Don't fight it, don't get mad at it. Just notice it and ride it out. Your brain has to severe the neural networks that were dedicated to him, while rebuilding new ones. This is a process that takes a while.
Start connecting with friends that are healthy. They can be a nice source of validation, connection, and support.
Work on a new project to have a focus. This can help in those moments where I'm sitting around ruminating with nothing to do or no desire to do anything. Even if I'm ruminating while doing the project, at least I'll something to show for it when it's over.
Start a new hobby to define myself apart from the relationship. I'm going to be a new person.
When ready, start throwing out all of their stuff. I even get rid of gifts. If it reminds me of then when I look at it, it's gone.
Things will get easier as you stop thinking about them slowly over the next dew weeks to months. Eventually, they become someone that you used to know with no real meaning other than the lessons you learned from that experience.