I would just keep telling myself they've lost someone who cares about them and would have given them lots of love. All I'd have lost is someone who doesn't care about me. I'd try to keep my mind focused on things other than him. It's pointless wasting feelings , emotions and even thoughts on a person who doesn't value them
Feeling bad about yourself after a crush doesn't work out is normal, but it's not a great way to feel. Taking care of yourself is a way to show yourself that you do matter, and it will also make you feel a lot better.
Get regular exercise, meals, and sleep. Get therapy if you are feeling depressed or anxious. Stick to a healthy routine every day. Clean up and dress yourself nicely, even when you feel like sludge it will make you feel better.
Delete all contacts with the person. Erase from all social medias and phone. If you know he/she likes to be a certain place hanging out avoid it. Distract yourself with new hobbies. Go to the movies with an old friend , exercise, play some video games , draw , listen to new music , look for a new job. Start fresh and move on.