I don’t know how you ended things with them, whether you had a conversation with them or just stopped messaging. Could your frustration be die to the fact they don’t seem to care about you as you did them? A conversation where you discuss things may be helpful, but that of course depends on whether you want to have it, whether you think the other person(s) would be receptive to such a conversation, etc.
In terms of “moving on”, other than perhaps having a conversation with them to bring closure the only thing that is going to help is time. It’s natural to feel upset, you need to gove yourself time to come to terms with the situation and process your emotions. If you find yourself dwelling on things, a good strategy I find is reading a book or doing something else you enjoy to take your mind off things. Is there anything you enjoy doing that could take you away from the situation and give you some respite for a bit?