Although I'm not a proponent of this, after my youngest brother died, all of us (my parents and sibblings) needed a little medical help.
Why don't you go visit your doctor. He might be able to give you something to take the edge off. I'm not talking about getting high, stoned or drunk. Just a little help. There will stll be tons of tears, but the feeling of your heart just exploding out of your chest may disappear.
Pease don't beat yourself up. It happened very fast. I had my Brindle at the vets that day she died from hemangio. They still waited to see if meds would work and tried a test and waited. Did something else and waited. So, the outcome for Otis probably would not have changed.
But wherever he is now, he's probably fretting that you're so upset, when everything is fine where he is now.