I'm sorry for your loss. My take on a lot of things is that as long as you do the best you realistically ask from yourself in a situation, you shouldn't blame yourself. Sometimes this is not as much as you'd like it to have been, and maybe you even missed the mark by a lot, but you do what you can and no more, and it will have to do.
My dad also passed away when I was in my 20s, and it was probably every bit as surprising as in your case, although I wasn't there to witness it. His behaviour wasn't always all that healthy, but it was still a huge surprise. Many would say it doesn't get easier and you instead learn to live with it, and for a long time it felt like that, but I guess it has also got easier somewhere along the years for me at least. Some part of it is still having learned to live with it, but I guess it's also kind of easier - but what's the difference between the two anyway after some time?
Finally, a quote I quite like: 'Our greatest glory is not in never failing, but in rising up every time we fail.'
