Im so very sorry. Please don't second guess your decision. Yes, it's very painful for us emotionally. Nothing you can do but grieve in your own way. Cry all you want to. I'm sorry that in witnessing his final moments it was so tramautizing for you. After watching my Vet euthanize a horse that had been severely burned in a barn fire, I decided then and there that I would not participate in that aspect with any of my horses in the future. And I haven't, as I know my limits.
I wish that I could wipe those images from your mind to help ease the pain you are feeling. All I can tell you is that, with time it will fade. Remember him by thinking of all the joy he gave you. Someday you will be able to think about him once again with a smile.