A man walked by an elephant trainer and was shocked when he saw no fences or chains to contain the massive animals. Rather, each one had a very small rope tied around one of its front legs and that was it.
When he asked the trainer why they don't just break the rope he said, "This is the same rope we use to contain them when they are very young. At that point it is strong enough to contain them. As they grow up, they are conditioned to believe that they still can't break it, so they never try."
In the same way, is there something that you don't believe is possible simply because it didn't work before? Is there something that you think is unattainable because of a previous conditioning?
Whatever you can conceive and believe, you can achieve.