"What you can do is often simply a matter of what you will do,"
~Norton Juster, The Phantom Tollbooth
My belief in this sentiment (excepting the few environmental or external factors that may genuinely constrain behavior such as war, extreme poverty or illness, for example) is one of the reasons I get so frustrated when people are unwilling to just do. Often, I hear people tell me "I could never give up meat," or "I could never give up dairy." I mentally shake my head and think, "Oh, yes, yes you could, yes, you can, and you would be so much better off." Maybe next time, I should respond with Thomas Edison's quote, "If we did all the things we are capable of, we would literally astound ourselves."
Conversely, I've always believe that if I wanted to do something, I could do it. This is a photograph of my room when I was a teenager. Ignore the Jim McMahon and Chicago Bears paraphernalia. Instead, take a look at the green sign: Of course I can do it, the question is: do I want to?
What is it that you WANT to do, that you CAN do, that takes nothing less that you DECIDING to do it?