OP, your story literally sounds like what I went through not too long ago. This is what you need to do: Drop her. Once you stop calling, texting, etc. She'll instantly forget about you. Don't do her anymore favors either. Once you stop all that, the words "best friends" will never come out of her mouth again. She says that because you go out of your way for her and she doesn't want you to stop.
Wanna know what I did? After I realized all that, I went overseas on vacation and hooked up with a girl while I was there. Got over the girl that used me and I've been good ever since. I'm still friends with her. She'll text me occasionally wanting to hang out or grab a bite to eat. And I do but I never flip the bill or pick her up anymore. I've realized I'm way better off without her. She's been dating different guys on and off since. I'm glad I'm not in that situation anymore.
Not putting miles on your Ferrari is like not f*cking your girlfriend so she'll be more desirable to her next boyfriend.