Katherine Jenkins credits her successful marriage to remembering to be her “husband’s girlfriend”. The 44-year-old singer has been married to Andrew Levitas for a decade and she’s been guided by advice from an older couple to maintain the “fun” and flirtiness in their relationship. She told Britain’s HELLO! magazine: “We have friends, an older couple, who are happily married, and I often ask them for advice. “I remember him saying to me once, ‘Make sure you’re always your husband’s girlfriend – meaning be that fun person, go out on dates. That doesn’t have to stop just because you’re married. …