Why Such Awful Pain Now? Options

[Furry's mum]

It has been 7 weeks today since my darling cat Furry had to be pts. I felt that I was beginning to cope a bit better. Have been back to work for a week & have been able to function, just.
Today was the first time that I didn't "observe" the time of her death, by looking at photo's or sitting by her grave & talking to her.
Later my OH, without my knowledge, decided to light candles around her grave, & then took me into the garden to see.
This made me feel like I did the day she died- terrrible grief & unstoppable tears, I still can't stop crying. I feel like I killed her - too much medication? or not enough? I can only think of those last painful minutes of her life.
Why have I got to this stage again, when I was beginning to think about the happy times we shared? She was only 12.
Please help me to understand why I feel this way now.
Judith