Sorry to hear this. Loved your pictures in a previous post. This is so tough to take, I know having been through traumatic times myself with a beloved previous mare.
At least you've tried everything, and believe me, in years to come that makes a big difference in getting over it.
I had missed the original posts so have only just read about your poor horse.
I really don't know what to say that can help to ease your pain and sadness at this absolutely horrible time, except to say how truly sorry I am