For me, Christine, I really found it helpful to follow up a shared interest. The fact that it was an interest I had shared with the person I had lost somehow helped make it feel ok to join a class. It helped also that this was a class you could physically attend as it was before lockdown, but I am sure even Zoom attendance would be better than nothing or even working away on your own as it will still take your mind off your grief for a while.
I think we all have that experience of going over all those things that happened close to the end going round in our heads over and over. I found I had to make an effort to move out of that. It helped to make an album of favourite photos and that did help re-evoke memories of better times too. It also helped to revisit places we had been to together as this also reminded me of better times.
The other thing is to accept that whatever you did at the time, you did for the best. We all do the best we can with the information we have at the time and even if we feel we could have done better looking back, that is not so. We don't have the benefit of hindsight at the time, so we just try to do the best with the information we have then. We can all find something to criticize ourselves for, but there is no one who will not have done what they thought was the best in the moment So if you are going over everything because you are feeling like this, please don't blame yourself. I am sure you should not be.
The other thing to remember is that the end of a life only makes up a tiny fraction of that person's life as a whole and it sounds like you and your husband were very happy together, so hold on to that. Illness and painful memories always surround the end of a life, but they don't blot out the many years of happiness and laughter and fun of the many previous years, so you have to remember that and be grateful and celebrate that too. They are much more important and significant than those painful last days.
Sending you a hug.