A small question: How old are you? You say that you liked her in HS, and that when she sent you a message in Messenger, your interest was immediately sparked up? You may not be at a place, emotionally, where you're ready for a relationship. The level of intensity you show in expressing, for lack of a better word, hatred, towards this girl tells me that this relationship definitely shouldn't happen.
In general, girls do not go out with guys that wait months before asking them out. It is very likely that she simply viewed you as a friend and thought that the feelings were mutual. She most likely was not being an attention whore. In fact, you seem to have made the mistake of thinking that there was some relationship between the two of you.
Unfortunately, waiting a few months before asking a girl out, only to get turned down, does not constitute any kind of relationship. She was under no obligation to you for anything, and you probably scared her a bit when by, "pressing the issue". I would not be surprised if she got herself a boyfriend for security reasons.
In any case, it is all a moot point. You're just out of High School, man! Fresh of the presses. This is not the time to be getting into relationships. Have you been to college? Take a course at your local community college. Try out some new things. Get to know yourself. Nobody has it together just out of HS. Work on improving your-self and your self-esteem. Oh, and forget HS girls. No man in his right mind should ever pursue a serious relationship with a High School girl. It's all just puppy love and other heart-wrenching nonsense.
Don't worry. You got your whole life ahead of you.