Maybe this is a dumb thing to say, but maybe Alex changed toward you because she thought you didn't feel the way she felt for you. It's obvious to me that she liked you a lot!
If I had acted that way with a guy who I thought liked me and then he never acted on it, I would have backed off too. Hopefully you'll get another chance with her. Yet, don't let the past blind you to others who you may meet who could be even more right for you. You will eventually find the right person for you. I know people say that a lot, but it does happen. I was almost 26 before I met the right person. We've been together almost 10 years now!! (Maybe you're looking in the wrong place, because I met him online!)