"Whether you believe you can, or believe you can't, you are right." Abraham Lincoln
There is nothing like a good dose of common sense with which to start your day and this wonderful quote fits the bill perfectly. Let me take it a bit further to see if I can help put it into context. It's not saying that you can't have some initial feelings of doubt when it comes to a challenge. Self-doubt can be healthy and keep you grounded in reality. However, the difference is that once you experience that feeling of self-doubt, it should automatically trigger the intellectual curiosity in you, as opposed to fear, and you begin to disaggregate the challenge and look at each aspect of it calmly and assess what needs to be done step by step. Now you're opening the door to gaining the information that will bring the challenge firmly into the "yes, I can" column. So, when you are next faced with a significant challenge, take a deep breath, let it out slowly, engage your mind, and move forward in incremental steps toward the solution and that is how to start that wonderful chain reaction of, "yes, I can".
Good luck on your journey!