@LucyintheSky21 Yes I know exactly what you mean and I feel the same. My dad was on holiday with us in Wales just 2 months before his diagnosis, enjoying a walking holiday. He was a bit out of sorts, but was out and about and getting in some lengthy hikes. Within days of getting home his legs were buckling and he felt ill, and after a misdiagnosis a new GP sent him for an MRI and they found the massive tumour, and that was it. He was 75 then. All the new walking stuff he'd just bought, and that he had asked for for Christmas, all unused as he never walked unaided again - when he should have been enjoying another 10+ years of active life.
You had the sudden loss of your dad, and that must be so hard to come to terms with. I knew my dad's prognosis, 2.5 years before I lost him. I don't know if that was better or worse - but I do know I did some of my grieving back then, when I lost the dad I knew, knowing he would never come to Wales with us again, would never come to my home again, I'd never sit next to him in his car singing along to the one particular song he loved and used to crank up to full volume in the car. But then we had the not-knowing, never really knowing when the end would come, I sometimes felt like we were always living in dread.
I've often wondered if that knowing the end was coming was worse then none of us knowing what was to come, it felt like such a long time to be grieving. But then what you're trying to comes to terms with sounds so hard. I wish I could say something to help, I really do.