Firstly, congratulations on your nearly 16th birthday! I can (just about! lol) remember what was like being 16 (Im 23 now).
Second, lets talk about your problem. I can understand what it is like to lose a grandparent to cancer. I have lost 2 now. I was around 12/13 when I lost my paternal grandfather to bowel cancer and I was 21 when I lost my maternal grandfather to lung cancer. I will not pretend to undertstand Fibromyalgia but I know odds and ends from what I have learned on here. Keeping your feelings locked up inside is dangerous... they all have to come out sometime and if you lock them away, they often fester and when they come out, it is out of proportion. Talking to your school councillor is a great place to start. Talk to your mum as well. Strike up an honest relationship where you tell each other everything and you can bounce off each other.
Feel free to talk to us as well. We will always be here if you need to vent.