The optimist underestimates how difficult it is to achieve real change, believing that anything is possible and it's possible now. Only by confronting head-on the reality that all progress is going to be obstructed by vested interests and corrupted by human venality can we create realistic programmes that actually have a chance of success.

This quote is saying that optimism can be misleading when it comes to making real change. When making plans or strategies to create change, it’s important to take into account the difficulties that will come with implementing them, such as those caused by people or organizations who are against the change. It’s only by fully acknowledging and understanding these obstacles that realistic programs with a chance of succeeding can be created.