Why can't I get over this?

I think that you're more bothered by the "what-if". What would have happened has you and Jack every actually hooked up. Even though you have never actually dated your behavior implies that you still want to find out what all the hype is about with him. It sounds to me like Jack is going out of his way to annoy you and keep your thoughts concentrated on him and why hes doing what with regards to you. It's a clear sign of jealously on his part. If you honestly want him to stop annoying you than stop letting him annoy you. He cant delete you from social networks if you don't allow him to to befriend you on them. You have a great guy and you said he makes you feel special and beautiful right? Trust me you don't want to give that up for a "what-if" whose probably not even worth the trouble. You get over it by not letting him and his antics with other girls bother you. You are in a relationship that makes you happy; why you should concern yourself with what Jack is doing? No other girl should have the power to make you feel insignificant. As long as you know that you are important those other girls shouldn't matter to you at all.