Peter Vander Wall argues that breaking promises to yourself is mental weakness. You keep promises to others for social reward, but skip your own. Each broken self-promise erodes self-trust, which he calls the core of confidence.
Chapters
Signs that you're mentally weak, you don't keep your promises to yourself or to others. Now most people kind of get it that you're supposed to keep the promises you make to people but they don't include themselves in that. We inherently want to look good and have people like us. One of the best ways to do that is to make promises and keep them. But we don't think the same way with the promises we make to ourselves. I'm going to get in shape this year. I'm going to start a business this year. I'm going to do this, I'm going to do that. You have to understand these are promises you're making to yourself and by not keeping them you're telling yourself you're a liar. Fun fact. The word confidence actually has a root meaning of intense trust and that's exactly how you build self confidence. You build trust in yourself that you're going to keep the promises you make.