You might know Kate Mulgrew as Galina “Red” Reznikov on “Orange Is the New Black,” Capt. Kathryn Janeway on “Star Trek: Voyager,” or perhaps as Sam Malone’s girlfriend-who-wasn’t-Diane-or-Rebecca on “Cheers.” For all her talent as an actor, however, Mulgrew’s true calling may be as a writer. In her new memoir, “How to Forget,” Mulgrew shares a moving personal story that’s a scriptwriter’s (and actor’s) dream. It’s full of love and heartbreak — of affairs and deaths and lives undone — as melodramatic at times as “Ryan’s Hope,” the soap opera Mulgrew once starred in. That the book is autobiography makes its many shocking details all the more powerful. When Mulgrew was a teen, for instance, her mother confessed to her that she was having an affair with their priest — a relationship she ended only when Mulgrew’s 12-year-old sister was felled by a brain tumor.