I cried when I read your post about his passing. Please don't feel guilty. You were on top of everything you could, taking care of him obviously a priority for you. Sending him to a vet school for losing weight would have been premature, and it sounds as though if it were cancer, there was nothing that could have been done.
I am so sorry for you loss. I had to put down my German Shepherd in March, and like you I second guessed myself. He had cancer as well, and I did the "I should have...." You can beat yourself up over it, but the truth is...you loved him, and it sounds as though you gave him an amazing 10 years! I am again truly sorry for your loss. It's hard to lose the ones that are once in a lifetime horses. (((Hugs)))
"I'm an idealist. I don't know where I'm going, but I'm on my way."
I am so sorry for your loss. I lost one a month ago ,and it is really hard. I still cry almost every day.Going to the barn is sad.I know I could not have done anything more for my guy ,and I think the same is true for you.It sounds very likely that he had cancer.I hate it when we lose the good ones.
You would have beat yourself up both ways and second guessed it forever.
You tried and did your best and put far more effort and money into it than some folks would have. Rest assured that the ending was predestined and that he passed away overnight in his stall. If he seemed fine before you left him then that was his gift to you.