This is a difficult situation. There are many things one should be angry about, but one has few options to deal with it: either you can let it go and move on, actually try to do something about it (even if you fail or it takes a lifetime or not always possible), or just stay angry.
I’m not going to tell anyone how to feel, but I personally would either let it go or try to do something. I’ve lost people before and I refuse to feel that shitty all the time; it is hell.