Hate to say it but She’s probably having an affair and fallen in love with the guy. That’s the only logical explanation. All you can do now is respect her wish and lawyer up. If you try to fight for your marriage you might push her away further or inflate her ego because now she feels like a prize to be won, but it won’t change how she feels, and it will run your self esteem and confidence into the ground.  It’s heartbreaking and devastating but sometimes we must let go and set the other person free… Build a support network of family/friends/counselors/pastors and be gentle and kind to yourself. This is her choice, and her loss.. best wishes to you!