Firstly, sorry you are going through this. you are not alone. But you know to do the right thing.
Secondly, if you go that route, have the hole dug, and the horse put down near the hole and buried before rigamortis sets in. Doing it in a stall brings up a whole lot of complications.
The lying down and unable to get up is often a sure sign … they are too big and have heavy organs that makes any length of time lying down helpless very hard on the horse.
Make sure you know the local rules and are not near wells, etc.