I actually came into this board to talk about my own recent berevement, read your post first, and couldn't not comment.
Of course you can't accept it. I feel the same way and my gran's death was imminent, but still shocking. I can't imagine how you might feel when unprepared.
My gran's funeral is next week. We were close so I've been asked to be a pallbearer, deliver a eulogy on behalf of the family, then lower the coffin at the cemetery. I honestly don't know how I'll manage to do all of these things without crumbling into a weeping mess. Like you, I just can't accept that she's gone.
The thing is, we just have to, don't we? There is nothing else to do but get up, make breakfast and get on with the rest of the day. I'm lucky in having a beautiful, smiley 7 month old who laughs all the time. He's keeping me going. If I didn't I'd need to find something that could distract me.
I wish I had wise words for you, but I don't; just empathy and the knowledge that the grief is overwhelming and insurmountable now, but you will get through this and the pain will start to lessen, as everyone tells you it will. We both will.
PM me if you need to vent and can't say it out loud to anyone in real life - all the words seem to get stuck in my throat, but writing helps me, and it might help you x