Are you a "yes" person? Are you constantly feeling stressed out? Are you always behind on projects? If you answered yes to these questions I have a feeling you are an over promiser or a yesaholic.
It's okay to say no. And it is okay to take your time on a project. If a client wants a new Web site and you estimate it is going to take 4 weeks to complete how about giving yourself a break and telling the client it will take 6-8 weeks. This gives you more room for creativity, things that might come up, technical issues, etc. Now if you actually do finish the Website in 4 weeks your client will be delighted that you got it done early.
What else are you saying yes to? Think about it. Be conscious and aware every time you say yes (or no). When you are saying yes to volunteering at your kid's school what are you saying no to? When you say yes to doing a last minute project for your client that is going to require you to work over the weekend what and who are you saying no to?
Tip: Every time you are about to say yes to something stop and ask yourself what you are saying no to. If saying yes to doing that last minute project for your client means saying no to the date you had planned with your significant other... maybe you want to say no to the client. Or tell the client yes I can do that for you and have it ready by Wednesday (not Monday).