My worst habit would be procrastination. I tend to put most things off until the last minute or deal with challenges once they border on being out of hand. This has failed to serve me well in all areas of my life. There have been times when I knew and understood that procrastinating would not end well for me.
Now let me also say that there is an upside to procrastination. When I am working academically, procrastination has proven to get the best work out of me. I feel that is because in this instance I am able to complete whatever I am working on without second guessing myself or feeling that I need to make major changes. However this is a study habit that has been perfected over many years of being a student. I would not advise anyone to try it.
I have also heard the saying that Procrastination is the thief of time. I can see where that has some relevance. I have seen procrastination keep me from doing things and holding me back from my goals. As I grow and mature and as I complete my educational endeavors I am learning to procrastinate less and be better at time management.