Welcome here, I think Startingnew has some good ideas. For some reason, I don't know why, it is very easy to be unhappy with what one has done and feel guilty and at fault as a result. Equally it is very difficult to acknowledge the good one has done. I found this when comforting an offspring whose partner had died unexpectedly.
I felt events were sweeping me along and I was pretty powerless to make things better. It was only later I found how significant my efforts had been.
I would think that there would have been so many things you said and did whilst nursing your mum that would have made a great difference to her at the time - and in all probability been a comfort to your father too. I'm not sure a longer stay -even a very long one -would change how you felt about yourself. Inability to fix things spills over into self-blame, even when totally inappropriate.
Anniversaries are a real trap, the only good thing is you know in advance when they are going to occur, so steps can be taken. Is there some favorite or enjoyable activity you can use to occupy your thoughts on the day?