Stephanie Szostak has held her own alongside A-listers in dozens of films and TV shows — but behind the scenes, she was dealing with crippling imposter syndrome. In her new book Self!sh: Step Into a Journey of Self-Discovery to Revive Confidence, Joy, and Meaning, the A Million Little Things alum, 48, opens up about navigating “toxic” inner dialogue on set. “It first started with self-doubt, and then it was physical too: My heart beating and my legs shaking, and just really convincing myself that I didn't belong here, that I was a fraud, that I was going to get fired,” Szostak exclusively reveals in the newest issue of Us Weekly of working on set. “It didn't help my performance, and it didn't really help how I felt.” Santiago Felipe/Getty Images According to Szostak, it took a few years of struggling in silence before she learned specific practices — like meditation and journaling — that helped her cope. She also started jotting down "takeaways and reminders" from books, podcasts and moments from therapy sessions and compiling them into a personal guide to refer to when necessary. “It’s my own little tool,” she explains. ‘A Million Little Things’ Cast’s Dating History: Inside James Roday Rodriguez, Romany Malco and More Stars’ Love Lives Szostak’s personal guide inspired Self!sh, a new workbook built around eight self-reflection exercises that will be released on World Mental Health Day and benefits mental health organization Give an Hour. “The point is for you to create your own personal playbook for life,” she explains, noting the guide isn’t solely about her own experience. “There's a story from a veteran, a champion boxer, a documentary, people who are in recovery, [dealing with] PTSD.” Post Hill Press For Szostak, positive thinking is a crucial form of “mental fitness” — which is why her playbook is the first thing she turns to when starting her day. “Most of us in the morning, we turn our phones on and we get bombarded by the news, text, emails — and all of it affects how we feel,” she tells Us. “And we're so busy, [that] a lot of times in a moment of need, all this inspiration or motivation that we come across doesn't make its way through the top of our mind [so] we get stuck with the same unsupportive thoughts.” The Satisfaction actress further explains that the book is “ all about self-talk, and self-talk is where confidence comes from.” “We all have that negative self-talk that creeps in, and it's about being aware of it and then creating your own epic thoughts,” she says. Stars With Anxiety: Celebs Open Up About Their Struggles and the Methods That Help Nowadays, Szostak has found a silver lining in her challenges — and she’s hoping Self!sh will help others look at things in a different perspective, too. “When we go through something that is hard for us," she adds, “it's also an opportunity." Self!sh: Step Into a Journey of Self-Discovery to Revive Confidence, Joy, and Meaning will be available on October 10. For more on Szostak’s own mental health journey and her new book, watch the video above and pick up a copy of the latest issue of Us Weekly, on newsstands now. With reporting by Christina Garibaldi