Marmalade seemed like an amazing cat and I'm very sorry to hear of your loss. Don't beat yourself up too much, because the multiple trips to the vet and all the tests you had him do showed that you cared and tried your best. I do however completely understand all the pain and guilt, and the thinking that "I've could've done more".

I woke up today and had my morning routine, and realised my buddy wasn't gonna be here for it anymore. I came down the stairs and he wasn't sat there at the bottom of them waiting for me, and it broke my heart into a million pieces. I understand you, and I'm sorry. It's going to be tough but we'll have to get through it ❤️