Deb, My heartfelt condolences are with you. We just recently put our sweet horse Annie down 3 months ago suddenly due to a shoulder break. If you search here, you will see the thread I started when we lost her. So I feel your pain. It is a difficult thing to go thru, but I am happy you don't want to see your horse suffer. You are a brave and selfless person.
As someone who grew up with horses, and have boarded horses at our barn for the past 20 years, I have experienced what you are about to go thru many times. Both with our own horses and those of our boarders as well. Some were planned, some were not, some were very sudden.
It is my VERY STRONG OPINION, to give them an injection when their time has come. It is the most humane way without a doubt. If you have an experienced vet, then this is the kindest way to put them down. First, they give them an injection that is a tranquilizer, this calms and relaxes them. We give our hugs, kisses and say goodbye all the time while stroking them, and then the vet gives them another shot that is an overdose that just puts them quietly to sleep. It happens very quickly and they feel nothing other than what you would feel going into a deep sleep. You hear their last couple of breaths, and then they are gone to be free, running with the horses in the big green pasture in the sky.
I have also experienced the gun as a way to end their lives. It is HORRIBLE!! A sight that I will never forget. The sound of the gun I can still hear in my head to this day, the sound the horse made, the grunt, the loud thud as they fell. That was with one shot between the eyes. Imagine, it doesn't always end with one shot, it may take a couple, how HORRIBLE for the horse that you love so much to experience that pain and suffering.... I didn't even know this horse, I can't imagine if it were my loving pet I was witnessing.
I have also seen many a horse whose adrenalin has kicked in and they were being put down by injection, they are already calm from the tranquilizer, so they felt no pain or suffering by having more shots administered to be put to sleep.
IN MY OPINION, IT IS THE ONLY KIND WAY TO PUT YOUR HORSE DOWN.
I am sure you will do what is right for your horse. Please keep in touch, and I would love to see pictures of her if you have any to share. We also cut a snip of her mane, put a ribbon around it for a remembrance.
You are in my thoughts and prayers.
Penny