In Parshat Toldot, Isaac wants to give the special "birthright" blessing to his son Esav. Rebecca, however, knows prophetically that Esav is undeserving of such a position, so she has Esav's twin brother Jacob disguise himself and receive the blessing instead.
Afterwards, when Isaac realized the switcheroo - that he had indeed blessed Jacob - he began to tremble greatly. Isaac trembled even more than he had years earlier at the Akeida, when he was bound on the Altar ostensibly to be slaughtered.
Rabbi Chaim Shmulevitz explains why Isaac trembled so greatly. It's because he realized that for all these years, he had incorrectly judged Esav as being worthy of Jewish leadership. All his hopes and dreams for Esav would now go unfulfilled. It was a shock to Isaac's system, having to adjust to the new reality - the truth as it now became known.
Let's try to appreciate how shocking this could be. Imagine you were living in the Soviet Union during the mid-20th century, as a card-carrying member of the Communist party. You read volumes of Marxist ideology, and believed all the propaganda that Communism will ultimately liberate humanity. But then, the Communist experiment proved a failure, and the Soviet empire broke apart amidst a flood of capitalistic spirit. How crushed you are!
Yet will you be willing to admit that you'd been wrong all this time?
All of us, at one time or another, find ourselves clinging to an old position, even when we know it's wrong. We have so much invested that it's painful to admit our mistake. But we need to realize that it's even more painful to go through life repeating that same mistake.
In this parsha, Isaac understands clearly that it was God's will that Jacob be blessed. Even at an old age, Isaac is able to change, to grow, and to move forward with the new reality.