Blake Lively admitted she made a mistake for mocking Kate Middleton prior to her shocking cancer battle revelation. On Friday, March 22, the royal, 42, shared she is undergoing preventive chemotherapy and set the record straight about her health crisis after rumors ran rampant about her whereabouts. “I’m sure no one cares today but I feel like I have to acknowledge this. I made a silly post around the ‘photoshop fails’ frenzy, and oh man, that post has me mortified today. I’m sorry. Sending love and well-wishes to all, always,” the actress, 36, posted to her Instagram Story that same day — jus…