A friend just put down a horse recently. I was there for the discussion with the vet. I found you have to give the vet enough information about your thoughts and limitations and definitely not try to have the vet decide for you in a situation that is not unquestionable.
This horse was very lame and had a number of concurrent issues. The lameness could not be cured. Still the vet offered some options that might buy him some months, but were costly and would add more stress to the horse. I think the vets feel duty-bound to present all treatments no matter how unrealistic. It must be a nightmare for them to have a customer later believe that the vet “told them to put the horse down” when there might have been a treatment. So in these situations, the owner has to be strong and clear.
I think it might be helpful to have an explicit conversation woth your husband about what conversation you are actually having and what kind of support you need.
It seems like you are having one of those conversations where it’s ostensibly about one thing but really it’s about something else.
The content of your conversation is [facts about the horse, logistics about the horse, option for the horse], and that is what he is engaging with, very literally, in his efforts to be supportive.
However I think the conversation you’re really having is that you feel you’ve exhausted all of your options, and all of your bandwidth and emotional and financial reserves, and it’s only downhill from here for the horse, so you want his support with the very difficult decision to put your horse down. First, you need for him to respect that as a life long steward of this horse, you are the expert when it comes to this. So, you’re not asking for help with logistics or making the decision (although I think perhaps secretly you were hoping he’d come up with, “what about euthanasia?” on his own so you could say “yes, i think so”, because you yoirself are having a hard time with it, which is 1000% understandable), you are asking for his support and comfort now that you have made it. Ways that he can be supportive could include calling the vet to make the appointment if you don’t want to have to do it, attending the appointment if you’d rather not go, taking the day of work to be with you if you do want to be there, brainstorming ways to help you remember your horse’s life positively, etc.
I’m not faulting your husband for not figuring this out on his own. It is a really, really tough listening skill to listen to “this vet said this, that vet said that, this retirement board option has this, that retirement board option has that” and come up with, “Hey, if you think it’s time to put her down, I support you.”
One of my friends once had a situation where one of her life partners was repeatedly picking fights with her about domestic or interpersonal things. Finally she had a light bulb moment and she asked her, “Are you really upset about how laundry chores are divided (or whatever it was), or do you just want my attention? Because we can sit and talk about the laundry and start solving that problem if that’s really what the problem is, but if you just want my attention then I’d much rather just have a cup of tea with you instead.” Her partner stopped and said, “Oh. I think I just want your attention,” and they had a nice cup of tea.
This friend clearly has NEXT LEVEL listening and communication skills, and mere mortals can not be expected to just do that perfectly everytime.
But, I think that example could be a good thing to keep in mind when discussing with your husband what support you really need from him. He is hearing “logistics” and quite literally discussing “logistics”. Most people can not just take the listening red pill like my friend did and realize they’re actually having a completely different conversation from the one they are actually having.
In bringing this up to your husband, I think, step one is for you to recognize and directly appreciate the effort that your husband is making to be supportive, because he is making good faith efforts to be supportive and help you solve the problem that your literal words say you are trying to solve. Then explain to him it looks like the conversation that you’re actually having is different from the one you’re actually having, and then tell him how he can be most supportive to you in that conversation.
Based on what you’ve written about your husband, I think if you build the bridge, he will readily, and lovingly, walk across it.
Thanks, we were able to talk about it after I spoke to the vet and scheduled the appointment. My husband had to step out to catch his breath but then came back and we wound up on the same page. He understands. We had to put down the only horse that was really ever “his” horse, a few years ago after a long day at the vet with colic. That pain is still fresh for him, he had never seen a horse PTS before, but she was in severe distress and already down.
It is just such a grieving process before and after and will be something we have to live with forever. The vet was very kind and supportive. I know so many more people have much harder things happening in their life, but this doesn’t make it any easier right now. Owning and loving animals always comes with a high emotional cost at some point, or many points, along our journey with them. No escaping this part but I sure hate it.
I’m so sorry you are going through this. No matter how much of the right thing or unavoidable thing it is in the given situation, it’s always incredibly difficult. I’m glad you are getting the support you need. And also that you and your husband are both working through this so that you can be supportive for him and his grief in this too.
You don’t have to be there to watch your horse being put down. If you have a friend that will be there and give treats or if the vet has an assistant to hold your mare that is a perfectly fine way to do it. You can explain to your vet that you watched your husband’s horse put down, and that it was too difficult.
Don’t let anyone make you feel bad if you can’t bear to watch, and want not to have the process be your last memory of your mare. 
I unequivocally support your informed and humane decision. ((Hugs))
