Why haven't I bonded with this mare?

Well it takes a while for a horse to become comfortable with you and have that "bond" What you are doing is right, just spending time with her and riding helps build up that confidence. She is young and is still getting used to you. Sometimes doing some ground work and talking to her so she gets used to your voice can help. Maybe greet her with a treat or something so then she has somethint to look forward to when you go see her.