The boss loves your work. Your spouse thinks you’re sexy. The kids—and even the cat—shower you with affection. But then there’s the Voice, the nagging presence in your head that tells you you’re a homely, heartless slacker.

Even people who appear supremely fit, highly successful and hyper-organized are sometimes riddled with debilitating doubts, fears and self-criticisms.

“Most people are struggling with difficult thoughts and feelings. But the show we put on for others says ‘I’ve got it handled,’” says Steven C. Hayes, a professor of psychology at University of Nevada-Reno. In reality, however, “there’s a big difference between what’s on the outside and what’s on the inside.”

Cognitive-behavioral therapy aims to help patients conquer their self doubts in two ways: Either by changing …