I tell my friend that I think she's a great friend but honestly, I don't really feel that way
Oh that sounds hard to deal with. Just how long have you been telling her that?
I've been doing it since we were young
It's good that you tell her she's a great friend but you aren't being honest with her. Instead of just catering to her feeling you should be honest and let her know how you really feel so she can work on being a better friend.
But it's okay for her to be a shitty friend to me, isn't it?
No it's not okay. If you don't like the way she treats you, you shouldn't stay friends with her. It' fine to end a friendship for that kind of reason.