I'm so sorry for your loss Princess. It's incredibly hard to lose someone you have loved your whole life, even when you're a "fully grown up grown up".
Of course it's not unreasonable to miss him as much as you do - he's always been a part of your life; you've never had to live without him before so it's a huge change, as much as anything else. It's wonderful that he was so loved, that he lived such a long life and that he had the chance to be not just a Grandad, but a Great-Grandad too!
I was incredibly close to my Nan - I used to call in to spend an hour or so with her on my way home from work every day and we'd spend every other Saturday afternoon sinking half-pints of lager in the pub round the corner from her house whilst we gossiped about everyone we knew! She died 18 years ago this month, when I was 21 and I was very much as you sound now - just lost and a bit, sort of... untethered - if that makes sense?
As for when it will feel better, I think it depends on the individual. I remember feeling very sad and tearful for the first few months, but I was able to remember, and talk about her without getting upset by Christmas - although the day itself had difficult bits because of all the associated memories, but they were all happy ones (or very, very silly ones!)
Even now, 18 years on, I still have moments when something embarrassing or exciting happens and I think "Ha! I can't wait to tell Nan - she'll LOVE this one!" swiftly followed by a sinking feeling, but then I love that she's still often the first person I think of to tell! She never got to meet DH or my DCs, but I know she'd have called them "my little scamps" or "you two mischiefs!" and that makes me smile.
It's OK to feel how you do right now - in fact it's normal to feel exactly how you do right now, it's when happens when someone we love dies and we just need to remember that the reason it hurts so badly, is because we loved them so, so much. 