Oh my... you wrote such a long post berating yourself with elaborate negative adjectives, and yet, you still did not explain how come you and not your ex partner ended up caring for the animal that she brought to you.

The cat clearly died a sudden, painless death. The vet told you so. There was nothing to be done. there was nothing preventable. My cats stayed for three hours in the bathroom when movers came in - many cats actually prefer tiny places and when cats are fostered (I have done some fostering), oftentimes it is recommended that they start off in a new home by being locked in the bathroom. So by itself, three hours spent in the bathroom for a cat is nothing. For you it would have been bad. I wonder if you anthropomorphize your cat. The way you described the look of horror on his face when you opened the bathroom makes me suspected that anthropomorphizing might be at play, at least partially. Usually, when a cat is locked and then the door is opened, the cat either quickly runs out of the bathroom not looking at anybody and then hides under a bed or couch, out of reach of humans; or, the cat does not move at all, staying where she was, or even moving further back into the corner of the bathroom.

I need to go to bed and I recommend that you repost in the forum about pets, to get a reality check from many cat owners. Something in your post sounds off; I have survived the deaths of many pets and on this forum many people posted about losing their loved pets, and I have seen a gamut of emotions others and I attach to that loss, but I have never seen quite an example like yours. when you write about flashbacks, they do seem real, and overall, your experience does evoke the image of combat PTSD. There is a forum about PTSD as well. Post there. You need to get help - your feeling of guilt and shame are out of proportion, excessive, and not congruent with the fact that you basically fostered somebody else's animal for 5 years during which only 4 months were bad but the rest were great and the end was as sudden as it was painless. You have excessive guilt and shame - that is for sure. Other than by posting on those specialized forums, help can be obtained by engaging a therapist. I do not know what would give you the fastest relief, but that you need relief badly is crystal clear. Please get help for yourself! You gave a good life to the cat - give it to yourself now!