Ali,
Please know that you and your family are in my prayers. Do not dwell on what you wish you had said or done. There is no way that you should put yourself on a guilt trip. I am sure that you loved your grandad with all of your heart, and trust me he knew that!
Remember the many wonderful memories of the time you had with him. There is nothing wrong with being sad (we all have things that we wish we could have done differently) but don't let anger fill you with negative energy. You need to eat to maintain your strength, so please do the right thing.
Take care,
EJ