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.
2004 BMW M3 6MT - B&B TriFlow section 3|AP V2 headers|AST 4200|UUC EVO3 SSK with DSSR|ZHP knob|Orion V4|Matte Black kidney/side grills|19" VMR CSL reps|245/275 Continental ExtremeContact DW|Track stuff: 9" square style 67|275/35/18 Nitto NT01|Schroth Quick Fits|PFC 08 & 01 Pads
2014 VW GLI 6MT - APR stage 1 tune|CTS Turbo intake & 3" downpipe|ST coilovers