She keeps breaking my heart, should i just let it go?

There's a girl who tells me how much i mean to her and tells me that she and i will be together when we both want it. she'll say these things one day then next tell me how she and her "on again off again" girlfriend are doing all these things and that she just wants to be with her. and i just don't understand. here i am faithfully and just willing to love her but she keeps doing these things to break my heart, and i'm starting to think it's purposely. The girl she talks to now, they've been together for a year and during that entire year that other girl has had someone else. and she knew about it, she was ok with being the girl on the side, but i'm trying to make her number one and i get shut out. it's a constant battle. she loves me, she loves her. she wants to be with me. she's still wants to be with her. i don't know what to do.... should i just let her go?