Why does this kind of thing always happen to me?

Because she doesn't know what she wants yet.. she isn't intentionally trying to fck you over. You need to wow her to push her over the fence. Show her why she should be with you show her that you're a fun person to be with and that you'll care for her. If you're persistent enough, she'll come around. Girls want guys who can prove themselves to her maybe you haven't done that yet.