But it's not the same, is it. I had 12 years since my John was diagnosed, and we too had nearly 50 years together. There's no answer. Your original post had 2 questions. The first was "how will I ever feel better about what happened to John", and it's a hard one to answer.
Since he died, 9 months ago, I have reached a level where I accept that his life must have been in so much turmoil, searching for words, confused, frightened, and exhausted, that death must have been a relief for him - and I have no idea if he was in pain, but unable to tell me. I convince myself that there's no Alzheimer's in heaven.
The part about half your life is missing is so true, poignant and heart breaking. But we've just got to make the best of it, and at least here, on TP, everyone understands. Personally, I could murder Real Life Friends, who happily chat about what repairs, decorating or gardening their husbands have been doing, what holidays they're having, how they "really need a break" etc.
But I'd rather they think that they can chat like this, without me being upset, than have to watch every word they say. It's not their fault that their lives have been AD free, no more than its our fault that ours weren't.