I am so sorry for your loss. It is so heartbreaking losing a loved one. Even when you know they're sick, and even in you 'know' it's coming ........ the reality of it actually happening is still so awful and such a heavy burdern.
I lost my Mum from Bowel cancer about 7 and a half years ago now, but I still miss her often. There are days when I just want to call her and tell her about my day or my week.
I know that it feels at times like things will never get any better, and given that this is still so fresh for you, that's not surprising. But three weeks isn't very long. This is a time to be very very gentle with yourself and just take things minute by minute. I would like to also suggest that perhaps you might consider getting some grief counselling if you do feel as though you are really not coping?
In the meantime, just remember that it takes some time to adjust to life without a loved one. And remember too that you are welcome here for as long, and as much support as you need. The BB forums are 'open' 24/7. And of course, there's lifeline; 131114.
Take care. I'll be thinking of you. xo