Setting yourself deadlines trips up a lot of people because they don't know there are TWO kinds of deadlines and you have to handle them differently:
Be aware of the two kinds of deadlines.
You can set hard deadlines for the things you can control. For instance, let's say your goal is to write the first draft of your novel or to reach a certain weight level by a certain date. As that date gets closer, if you're behind schedule you can adjust your efforts accordingly.
The other type is target deadlines, for things not totally under your control. For example, let's say you want to land a new job by the end of next month. There's a lot you can in terms of applying, asking friends to keep their eyes and ears open, sending out resume, etc.--but you can't force someone to hire you by the deadline.
For these kinds of goals, target deadlines help make sure you are working hard to make thing happen, but you have to accept that you'll have to be flexible. Many people get discouraged and even give up when they don't achieve this kind of goal by the deadline--and that's a mistake. You fail only if you give up!
(for more on how to set and reach goals, see the new expanded edition of my book, "Focus: use the power of targeted thinking to get more done." It's published by Pearson and available from Amazon and other online and offline retailers.)