There is a difference between "I can't do it" and "I don't want to do it".
Society in general seems to use these phrases synonomously. When presented with a challenge (such as juicing for seven freaking days...or go back to school...or start running...) a lot of people's reactions are, "Oh...I can't do that."
Umm...how do you know? You haven't even tried. What you really mean to say is, "I don't want to do that."
Make your choice and stick by it. Don't flounder around and make excuses for yourself. Either you want to...or you don't. Believe me...you can do it. It just comes down to if you want to, or not.