Making a commitment to yourself (like a New Year's Resolution) and then breaking it may feel like the rule rather than the exception if it happens again and again. This lack of self-discipline can often lead to or reinforce a lack of self-esteem.
What's an easy way to improve your self-discipline? Pick something quick and easy to do every day, Make it a priority to do it. It could be as easy as waking up and saying the work "discipline" every day. Can you commit to something so simple for 30 days? What do you need to do to insure success? Write notes, set reminders, have a buddy to check in with?
When you've reached 30 consecutive days, maybe you could up the ante to something that would have more effect on your life (exercise, diet, etc). The goal here is to "strengthen the discipline muscle" that may have atrophied from lack of use. Like physical therapy, start small and work your way up.