Naturally all of this excludes you being abusive, neglectful, you cheating first, etc.. which if that's the case then you Are at least partially to blame. But if you have generally been a good husband with no serious mistreatment of her, then the next part will hopefully help a little.
While not quite the same as a 28 year marriage, i think the general principles in what i'm about to say will apply. My last girlfriend, who i had hoped to propose to, well, i was really in love with her. I did my best to be as good for her as i was able. She talked about wanting to get married too. But in the end, she ended things. I spent 6 months trying to win her back. At first she refused, then she spent a few months on the fence about it. Now it seems to anger her deeply. All i did was try to love her.
Or many years ago i befriended a woman. She was in her 50s, married to a baptist minister and they had multiple kids together. Out of nowhere she began cheating on him. A lot. With all kinds of men, just flings, not even relationships. Her husband didn't mistreat her. They had a decent house, great kids. She was happy until one day something inside her just snapped and she went wild. Couldn't even explain it herself.
Sometimes people just change out of nowhere. I don't know if it's fear, or boredom or something else. But there are two more cases where people, all women, i just realized, suddenly did a 180 on men who were good to them. So it's not you being a failure. It's just the way life goes sometimes.
Your wife needs therapy and counseling. It's good that you're separated for now.
