When you feel like you’ve learned whatever there is to learn from what you’re doing, it’s time to change course and find something new to learn so that you can move forward. You can’t be content with mastery; you have to push yourself to become a student again. “Anyone who isn’t embarrassed of who they were last year probably isn’t learning enough,

Don't worry about whether you can eventually become an expert (see Gauging mastery). Rather, try to constantly improve on the subjects you already know and seek out useful things to learn.