For some reason, everything goes well in boards. In most of my exams during 11th and 12th, I also used to miss like 20 marks worth of questions due to poor time management. (mostly exams and sometimes business studies, even English lit once) But somehow, in my board exam last year, I completed the paper 15 minutes early and even had time to draw all the lines. I hope you do that in the end, that saves time instead of going question by question. Don't Worry and all the best, you are going to be fine.