I know compelled how you feel. I lost my dad to lung cancer in April this year. We thought he was doing well and things were going his way. Until he was admitted into hospital with covid, he was very unwell but eveh day he powered through and got stronger. The day he was sue to be discharged he passed away very suddenly. Thank goodness I had this gut feeling. I am a nurse and and worked at the hospital so I went to see him. Although no visitors  were allowed they let me in. So thankfully he was not on his own. 
I am the oldest and have always taken on the more 'sensible' role and I guess being a nurse I was also looked upon to take the lead. I wouldn't ever change that, but it does put alot of pressure on, made worse when your trying to protect your family from the true extent of what is going on. Yet I am always let wandering what if I had done things differently. 
Nothing truly prepares you, it's always a process we are trying to navigate. It doesn't get easier, I think we learn to navigate around the grief.

I find talking about my dad.... reminiscing helps. Sometimes I feel like people have just stopped talking about him, like he never existed yet he has left such huge hole in our lives. My Aunty questioned why my mum had signed my brothers birthday card from 'mum and dad', and I replied that it came from me. When I had my first birthday without dad, mum signed it from just her. But I said, you can sign his name. Just because he's not here, doesn't mean he can't be part of it some way. He will always be my dad!! But maybe I am wrong, I don't know!! 
 

The firsts are the worse. Christmas will be very hard, just getting through each day is an achievement so be be proud of that. All the best