Alternate Reality

I'm 22, and my father was only 58:

He died three weeks ago, it was sudden - but also not? He had become extremely sick (heart failure), but was treated and had doctors telling us that now that they had seen so much improvement we'd have a few more years. Not even a week after hearing that, he died. It's been just us almost my entire life, and now I have to exist in a world that he's not in. Often I make jokes about him being dead, or something of dark humor. I feel like I do it not because I want to, but it's the only way I can cope without falling apart. Living in a world without him feels so, so strange. I've found myself dissociating a lot, it's like this reality doesn't feel real. I know he's dead, but some part of me thinks he'll call me or walk in the door tomorrow - even though I have his ashes. Why can't I fully grasp the fact he's gone?