I’m sorry for your loss. I know words won’t make you feel any better but know that you’re not alone, there are plenty that have gone through the same thing and unfortunately you won’t be the last.
Last week I also had my 21 year old put to sleep as he had developed a few different conditions (including being neurologic) but he’d had a recent personality change which brought it all to ahead. He had a moment where I saw him briefly stagger like a drunk on completely flat ground and he’d walk down ditches like a pacer, both left legs together and then both right legs. I was watching him quietly grazing in the paddock and in a split second he started running like a bat out of hell and then stopped suddenly 20 metres later with all four legs slightly splayed out. It wasn’t a normal bit of fun, he had a panic and started to bolt. In hindsight I think it was probably proprioception related.
He too had a personality change and became what I’d consider a danger to handle. He was unpredictable, going from leading quietly like a well-trained horse to being prepared to go right over top of you if necessary. He was on lookout 24/7 for imaginary monsters. I can only surmise that he was in pain or knew his mobility was affected and it made him nervous.
I like to say that we aim to give them a good life but not necessarily a long life, that part is often out of our hands. You gave him a good life and a humane end without suffering, that is more than a lot get.
Be good to yourself and take care.