Hi Buffy,
I know exactly how you feel. My grandpa died just a month ago and I can't believe that he's gone either. I know it's only been a month but I do know what you are going through, it's hard. I have some regrets about it all, like the day before he died (he died on father's day) I was working and my mom came in to my work to tell me that she was going to go visit him, and now that I think of it, I should have too after I was finished, but I didn't and now I wish I had, you know, just to say sorry one more time. All my family members tell me that he knew without a doubt that I loved him, and I'm sure he did but in my heart I wish I could just tell him one more time.
I guess all I can say Buffy is that, it's normal to feel this way and that it will take lots of time, especially if you were used to seeing him a lot. Just remember all the great times you have had with him and treasure them, because that's something that will never leave you.
One thing that helps me is I write my grandpa letters, not every day but some days, just to tell him what I'm up to, or how I'm feeling, basically just anything that I would say to him as if he were here. Also, try spending a day with your grandma if you can. That seems to help as well, at least it does for me.
Well, I hope I have helped you some. My thoughts are with you.