ssd - for me, that upside down feeling lasted several weeks, although at the same time I could feel the raw grief dissipating very very gradually. In many ways the death of a parent is life changing and nothing is ever the same again. That doesn't mean you'll never be happy again, just that the world shifts on its axis and nothing feels quite as secure as it did before.
My mum died on 18 October 2011 and I spent three very weird weeks immersed in grief and sorting out her things at my dad's, then had to return to London to start a new job. Nobody in my new job knew about my mum - I was functioning but not able to talk about it to anyone outside my immediate family. In those early days I read a lot of books about life after death and near death experiences in a kind of desperate search to work out where she'd gone.
Christmas wasn't great, nor my mum's birthday in January. I have a box of her things, and it was a good few months before I could even take the lid off to look inside. I felt more optimistic and hopeful as spring approached but I remember sometimes feelings of happiness turning into a feeling that I could never really be fully happy ever again because my mum wasn't around.
Summer was mostly OK, as most of the major anniversaries were over by then. I still thought about my mum every day, but I wasn't completely floored by the memories. Having said that, there were (and still are) odd moments when I privately collapse and call out for her and sometimes the longing to see her again is truly unbearable. In between these moments, though, I'm pretty much OK.
As I said upthread, I found the approach to the anniversary of her death very difficult. It was as if she was dying all over again. But now the one year anniversary has passed, I can say life is pretty normal again and has been for some time really.
Everybody seems to experience these things slightly differently, but there also seems to be a lot of common ground ... and one thing's for sure, you will find a way through this 