Welcome to Psych Kat, PC is here to help understand ourselves more clearly and find paths to working relationships.
I feel the emotions of a first love and wanting that person to be the one. I think you know that it is not practical but the emotional part is afraid to let go.
Virtual friends are easily seen as perfect because we don't have to deal with the annoying habits that people have in real life. You are only 18 so why not date some real people or just be friends with them.
There are no easy answers to relationship questions, but staying with the first person we have a crush on can limit our range of relationships. I don't think you cheated on him by going out with someone. You need to give yourself the freedom to explore. Committing to one person is better done with much relationship experience with several or many people at least being friends or dating to know each other better.
I think the way to get over that is to start living and working and preparing yourself to live in a whole new world of being an adult. You have survived the teenage years with extreme hope and crushing disappointments. Now you are ready to get into the next phase.
There is a teen forum if you care to visit
Teens' Lounge - Forums at Psych Central
and emotions
Coping with Emotions - Forums at Psych Central