So sorry to hear this but you absolutely did right by your horse. I have lost two homebred youngsters to colic and it is so sudden and shocking when it happens. If you are like me you will spend quite a while wondering if there was something that you had missed or done differently but very sadly sometimes awful things just happen. Oddly, I have a homebred horse called Bisto who is big and brown who has very recently done his first PSG.
You did the right thing. I think the suddenness of colic is awful and you have to make decisions when you really don't have a straight head but you knew him better than anyone and did what was right for him.
I'm thinking of you and also of Ol, who I lost the same way. Even eight years on, I still think about the "what if's" but I know in my heart I did the right thing too. I hope that knowledge will stay with you.