Sounds like it may be best if you moved on, since you have tried to talk it out with her multiple times, but nothing is changing and you are still unhappy.

Some people will keep someone on the back burner, so to speak. They will only come around when they need a "pick me up" or some attention. People like this will just randomly pop into your life whenever they need the extra attention, and once they are satisfied, they disappear for a while again. Sounds like that may be what is happening here.