Maya, do call back all these managers and ask WHY you weren't hired. Tell them you would like any constructive feedback they may have. I do this and it helps me fine-tune my interview skills. First time I did this - I was about your age - I was told by one interviewer I slouch too much. I was told by another I simply looked too young for sales, nobody would take me seriously. Etc. You never know what you will hear. You need to have tough skin, but it's well worth it. Also, if you handle the call professionally - resist the urge to get defensive or upset - they will remember you later, if something opens up. I have also had people give me additional job leads, in situations where they liked me, but the job just wasn't quite the right fit, or they hired a more qualified candidate.
Otherwise, all I can tell you is what I've been telling my DD, who can't find a job either: there are so many people out of work right now. You are competing against heads of households who have kids to feed, and retirees who've had to go back to work.
I agree with Angie, volunteering would be smart. How about volunteering to come in and dress the mannequins in the window? I don't know if this is some formula they get from corporate, but perhaps there's a small locally owned boutique you could approach.