Shannen Doherty is opening up about how she went into surgery to remove a brain tumor just after learning that her third husband, Kurt Iswarienko, had an extramarital affair. On the premiere episode of her newly launched podcast, “Let’s Be Clear with Shannen Doherty,” the “Beverly Hills, 90210” alum shared the devastation she experienced during a critical time of her years-long battle with cancer. “I went into that surgery early in the morning and I went in after I found out that my marriage was essentially over, that my husband had been carrying on an affair for two years,” she revealed. “To …