It's absurd when adults tell kids "you can do anything you set your mind to". What a lie. Every adult knows that there are some things he cannot accomplish no matter how much he wants to. In the typical example, there's only one President of the United States at any given time, and the vast majority of people aren't even remotely qualified for the office. Any kid who believes such tripe is destined for disappointment when he meets harsh reality.
What would be more helpful would be to tell kids, "you can try anything you set your mind to". At least it's true. Kids should be taught to try things, to take risks, and to accept failure. One of the most important lessons is life is learning when to give up. Many people give up too early, and some people give up too late, but an essential part of wisdom is knowing when enough is enough. Most people will never be President, no matter how hard they try -- and it should be obvious that the most qualified people don't even run for the office. Why? Because they're smart and they know that the path is long and hard, and largely a crapshoot. Running for President isn't a good investment of time, because it's likely to be a complete waste.
Whether it comes to relationships -- you know, the ex who just won't go away -- or school, or a career, or anything, giving up at the right time is a key to a healthy and happy life. Myself, I hate giving up, and I've written on the primacy of tenacity when it comes to grad school, but some of the smartest decisions I've ever made involved knowing when to call it quits.