you can actually run out of self-control at times

Psychologists have proven that Self-Control is an exhaustible resource. This is a crucial realization, because when we talk about "self-control," we don't mean the narrow sense of the word, as in the willpower needed to fight vice (smokes, cookies, alcohol). We're talking about a broader kind of self-supervision. Think of the way your mind works when you're giving negative feedback to an employee, or assembling a new bookshelf, or learning a new dance. You are careful and deliberate with your words or movements. It feels like there's a supervisor on duty. That's self-control, too. Contrast that with all the situations in which your behavior doesn't feel "supervised"—for instance, the sensation while you're driving that you can't remember the last few miles of road, or the easy, unthinking way you take a shower or make your morning coffee. Much of our daily behavior, in fact, is more automatic than supervised, and that's a good thing because the supervised behavior is the hard stuff. It's draining. Dozens of studies have demonstrated the exhausting nature of self-supervision.

The research shows that we burn up self-control in a wide variety of situations: managing the impression we're making on others; coping with fears; controlling our spending; trying to focus on simple instructions such as "Don't think of a white bear"; and many, many others. Here's why this matters for change: When people try to change things, they're usually tinkering with behaviors that have become automatic, and changing those behaviors requires careful supervision by the rational mind, as opposed to the emotional mind. The bigger the change you're suggesting, the more it will sap people's self-control. And when people exhaust their self-control, what they're exhausting are the mental muscles needed to think creatively, to focus, to inhibit their impulses, and to persist in the face of frustration or failure. In other words, they're exhausting precisely the mental muscles needed to make a big change. So when you hear people say that change is hard because people are lazy or resistant, that's just flat wrong. In fact, the opposite is true: Change is hard because people wear themselves out. And that's the second surprise about change: What looks like laziness is often exhaustion.
(via heathbrothers.com).