Am so sorry to hear about your Grandpa. Mine died 40 years ago and still there are some wonderful memories that keep him close in my heart. You also have tender and loving memories of your Grandpa. You will never want to forget that special bond you two shared. Have you thought of writing those down, as in a journal or notebook? Adding photos to them. It would be a way of preserving memories and also help you through the grief you are going through. Talk to people; family, friends, your clergy, school counselor, etc. They can provide comfort to you and help you understand that your Grandpa's passing, while very painful for you, is part of the natural process of life.
Right now you are going through a very tough time. I would like to say that it does not last long, but everyone is different. We do not all heal at the same time or in the same way, but we do heal. My prayers are with you and your family.
I will leave you with some resources:
Grief Support For Children and Teens -The Healing Place Helps When Dealing With Death and Loss of Loved Ones (read about understanding grief)
Dealing with grief and the loss of a loved one
Take care and God bless.