What should I do in this situation?

Ha, I've been in this situation before. This is my personal opinion: if you really like her, play even harder. It doesn't hurt to spice it up a little bit. Remain her best friend but try your hardest to fight for her! She'll notice something different. If it doesn't work out that way, continue to be her friend but try looking around, it's no fun hanging on the sidelines while she flirts with all the other guys. If she's flirting with you at all, there's hope for a better relationship. Don't bring it up until you start feeling a good change in the relationship, like it is moving in the direction you want it to.

i think you might want to try and see if she will even be open to anything more but if she says no then be ok with it and maybe try to find someone else... i would still suggest being friends if she says no that way when you are over her your relationship will last... i still find it very likely that she might say yes to you if you asked her to be more than friends but thats just my opinion

i think you should talk to her about your feelings. and even ask her out.
If she rejects you, just stops talking to her and save yourself in pain

Stay with her as friends. If you leave, it'll only be more painful. Flirt back, see if it goes anywhere!
Good luck!