Chocolate nut- I'm so sorry to hear about your mare. It's such a hard decision to come to sometimes but, once you've done all you can there is some peace that comes with the choice to euthanise them and end their suffering.
I would agree with KG349 and Justbloo about not actually watching your mare go down. I've seen hundreds of euths, dogs and cats are usually peaceful but horses can go down so hard, I would turn around once the vet starts injecting because the picture usually does stay in your mind and that is not the last thing you want to remember about your mare, I would say my last good byes once shes on the ground.

Even though I've seen many horses put down it was so different when it was my own.