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Being devastated is understandable. But after 12 years, why the sudden change with respect to the marriage? If you haven't noticed a problem, maybe there isn't one? I would slow down on the divorce. This is a big violation of trust, but 37 years of sharing your live with someone makes you far more important to him than 12 years of shacking up with him once in a while. If she meant that much to him, you would be divorced already. I would encourage you to work it out. Go to couples therapy if necessary.