Right - people act in a way that makes them feel good. It makes her feel good to have you around as a back-up. Why would she give that up? She has a great thing going right now. She has two guys so if one is cranky, she just turns to the other one.
You need to look out for you. You've already tried the talking route. You need to make it more clear and cut the umbilical cord.
I know it can be hard to find a good partner before university, because you're just "stuck" with a random assortment of people who probably do not share any interests. You'll find it is really different in university. Now you'll be with people who all chose that university for similar reasons. I bet you'll find females who are good matches for you there. Hang in there!
