I was very close to my mother and she passed away in hospice care in 2011. I remember being very devastated although most of the sad traumatic memories I've managed to repress, sometimes they come to surface but I will them away.
When my soulmate Beck died last year, I cried twice as much. I felt guilty too but Beck was my best friend, I saw him everyday, I took care of him, I hugged him everyday. He was the last thing I saw leaving the house and the first thing when I returned. He would even give me "goodbye" kisses - he would wait by the door when he knew I was almost ready to leave for work. I would pick him up and he would lick my face. The two relationships are very different so the way we grieve will be too. I don't think it's anything to feel bad about. I don't feel guilty anymore. My mom knew how much Beck meant to me ten years ago. He was also there for me through her sickness and death. He would wake me up with his familiar meow when I would get sleep paralysis. I think his death may have hit me harder too because of that, and because of prior loss it was sort of a layered grief. Maybe you are feeling something similar too. Sorry for the loss of your Monty and enjoy those lovely memories he has left behind for you.
“Until one has loved an animal, a part of ones soul remains unawakened” ~ Anatole France