Do I have a valid reason to feel used in this friendship?

You have done the right thing by telling her how you feel. Maybe she is just trying to fit in and find herself but these things might straighten out now the you have finished high school. If she continues to make your feelings towards the friendship seem wrong and she never sees or bothers to help the maybe its time for you to move on. Either that or she just doesn't understand and she cant help it. Are you doing anything that could offend her. Maybe you just need a small break from eachother but if you friendship is strong enough she will realise what going