Well....hate to break it to you....but it looks like you tried not just once..but MANY times and this girl told you the same thing each time. She doesn't want a relationship. I highly doubt its gonna change....unless she likes the attention and if you suddenly stopped it might peak her curiousity. But honestly this is highly unlikely....

I say stop pursuing her. She made it VERY clear to you that she doesn't want a relationship at the moment. I heard that one before and I found out that its a nice way of saying "Your a great person, but I don't like you like that." Then a few months later he was dating someone else. So don't be surprised if this happens....(it could).

The only thing you can do is move on. THere are other girls. Plus (at 15) you have years of dating ahead of you. Plus plenty of time as well.

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Appreciate the good times and learn from the bad times....