You are 15. So, concentrate on your studies and don't worry about losing all the weight that fast. I used to be fat as well, but over the years with intensive training and discipline, I managed to lose all the weight healthily and gained muscles as well. As for the girl, there will be plenty of girls out there. Don't put all your energy on the one girl that you think you will be with forever. Move on and start taking care of yourself properly. You yourself are the most important person in your life.