Of course your word matters to your clients and colleagues - while managers, marketers, and those who hold the budget can make idle promises, it's somewhat ironic that anyone with a foot in the creative realm really has to take care to deliver on what he (or she) says. Designers, producers, writers, artists - we are often thought of as being less grounded, less disciplined, than the suits.
But they expect us to deliver - and that's right, and that's fine.
The point today is different, but no less important. The point is to keep your promises to yourself. If you swore you'd never do X, then make sure you don't. If you said to yourself Y and Z would happen, move the world if you must to make them happen.
Treat yourself right.