Sometimes I have this brain problem. Usually my brain and I are pretty good friends, but sometimes the little fiend likes to play tricks on me. I know this because I've caught the booger snickering behind my back when it thinks I'm not looking.
I swear I'm not paranoid or anything. Let me give you an example.
I am perfectly capable of performing pull-ups. (Insert man-like grunting here. I know I'm not a man, but it's macho to grunt when you are talking about physical prowess.) On a good day I can complete five. On a really good day 6 (Okay, okay...5 1/2). That is on a good day at the karate dojo I can completely lift my bodyweight 5 times. However, (This is where the brain trickery comes in.) in spite of the fact that I weigh the exact same amount at the dojo as I do at the park, at the park I struggle through 2, sometimes 3.
You see, the pull-up bar at the park is higher up off the ground than the pull-up bar at the dojo. Now you wouldn't think that should make any difference, but you don't know how evil my brain can be. For some reason, my brain sees the goal (which is lifting my body weight in order to pull my chin up over the bar) as farther away. At the dojo, in spite of pursuing the very same goal, my brain sees the prize as closer.
Now for the moral of the story (watch out Aesop), a prize that is within reach is easier to achieve. Too often we set our sights on a goal that just seems too darned unattainable, so it's easier to give up. But when the goal is closer it's easier to stretch yourself just a little bit harder, just a little bit longer to grab that sucker in your hand and hold on tight.
So be sure you are setting believable goals for yourself. You'll be much more likely to feel the pride of accomplishment that comes with reaching that goal (or pulling that chin up over the bar).
I simply have to work on convincing my practical joke-playing brain that I can just as easily do pull-ups on the higher bar. My muscles are just as strong at the park as they are at the dojo (although I do look pretty bad-ass in my gi).