It is good to tell the truth, but be gentle about it. You did not give us enough information, in my opinion. How old are you? Have you tried counselling with your wife? Is there a chance of reconcilliation. My opinion for what it is worth, is to explore all the options of staying together, and then move out. Then I would live alone till you get some things setteled in your head, and then pursue this girl. If it is true love will stand the test of time. Also she probably needs to sort some stuff out too. Just don't be too hasty.

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Susan