When others criticize you or your work, it can be educational or it can be debilitating. Criticism or complaints, being negative, can be rephrased in your mind as constructive feedback. For example, if people complain that you have been burning the toast, you can rephrase that to mean that the toast should be cooked for a shorter period of time. In this way, criticism can be helpful--if the speaker isn't merely being negative because he or she wants to put you in a bad mood.
Criticism can also be debilitating. If someone is in the habit of saying negative things to you, it can paralyze you from action. Let me explain what I think happens.
1) Someone says a complaint or criticism.
2) The first reaction could be anger because of the manner in which the criticism was given, or because you don't believe the speaker.
3) Later, you wonder whether there is some truth to it.
4) Self-doubt creeps in, especially if you lack knowledge about how to correct the real or imagined problem.
5) You take it personally. You're the one who doesn't know how to resolve this, so there must be something wrong with you.
6) If there are enough unresolved issues, you procrastinate.
The first three steps are natural. Steps 4 and 5 lead to procrastination, so the solution must be between steps 3 and 4. What is lacking is information. Information can confirm whether the complaint was probably true or false. This is where it is helpful to ask other people their opinions on the matter. After asking two or three other people, you can see a pattern. Either the people you ask will agree with the speaker, or they won't. Only then can you decide objectively for yourself whether the remark is true for you or not. If it's not true, you can stop the process here.
If there is some truth to the negative remark, the next step is to decide whether to resolve the problem now or later. Generally, we all have too many issues and projects on our minds already. At some point, it becomes necessary to put many of them in the back of one's mind. You have the rest of your life to spend on self-improvement; you can afford to put most self-improvement issues off until later. In this case, you can stop the process here. However, if the problem is critical enough, it might be good to start working on it right away, at the expense of less urgent or less important problems.
Next, it's time to brainstorm solutions to the problem. Hopefully, the people you asked will also give advice, some of which might be invaluable. If not, there are plenty of self-help books, web sites, and internet discussion groups where you can go for more ideas. Then, brainstorm by yourself without interruptions. You can weigh the different options, and formulate a plan.
Finally, you can start learning new habits which will gradually eliminate the problem. By responding to the criticism with action, you can increase rather than decrease self-motivation and productivity. Just be careful not to try to learn too many habits simultaneously. Break each new habit into baby steps, and give yourself plenty of time to master each new step. Patience will help you to learn new skills, not impatience. As long as you know you're making progress, the negative feedback won't debilitate you.
One final note: If you spend too much time listening to negativism, it will overcome you until you can't help but procrastinate. I'm sure you know some people who spent too much time watching the news after a major catastrophe (11 September), and ended up not having the will to get anything done. Take steps to make sure you spend less time listening to negative people!