You have been dealing with alot, I wonder if your breathing problem is anxiety related?
Remember, God does not give you any more than you can handle, so you must be pretty strong.
I lost my mom pretty young, so I understand your worry, and all I can tell you is to enjoy your grandpa now and be sure to ask him about his life, his childhood and his favorite memories, so you can help carry them on when you tell your kids. It will help you feel like you know him really well! He may be around to spoil your kids, but if not, they can get to know him through you.