girls are bitches like that. don't be a back-up plan. accept that she doesn't like you enough to try to make something work with you and only you and move on. a girl who needs multiple guys in her life to feed her attention and boost her confidence is bad news. especially because if you do work out in the future, then you have to deal with knowing she has other back-up guys in her life for when things go sour with you and will probably end up cheating and lying and going behind your back for attention and all that other shit. you don't need that.
don't be another one of those rejected guys that she keeps in her life and uses you. those girls fucking suck. go find a girl that is willing to commit to one guy, and if that guy is you, great, and if isn't you, then you move on and keep searching.