I think there is often trauma associated with having to make the decision especially with a youngish horse with a chronic progressive condition. It is different with people as you don't hold that responsibility.
With Homey I still struggle, really I was calling Blue Cross pet bereavement support line for nearly a year until I got Bert and I still feel sad. I wrote the post grief after loosing a horse on here to try and also get support from here too. The world feels empty without Homey something is missing that cannot be replaced. You will get used it. It is normal to feel the way you.
So sorry for your loss Michen, that was a beautiful tribute and a selfless decision, it doesn't get easier but you do learn to live with it. That sounds miserable but hopefully you know what I mean, allow yourself to cry and rail at the unfairness of it all.