3. You broke up with your ex because you were having an affair with your current boyfriend.
“She told me that they broke up because of him and I believed her.” It may not reflect favorably on you to tell the truth, but it’s not fair to your ex either. You may also lose your current boyfriend’s trust if he finds out the truth. While there’s no reason to go into details about your past, if he asks, simply say that you’re not comfortable talking about it.