What's the cause of you losing heart and then not keeping to your promise to yourself to lose your unwanted weight? Could it be unrealistic expectations? Were you being overly optimistic about what you'd achieve in the first week? Did you think you could get there much faster than you really could?
Unrealistic expectations may give you the encouragement you need to start the weight-loss process, but it won't help you maintain your momentum. When you begin to realize you can't meet your expectations, not by a long shot, your motivation is going to slip and your commitment to keep on keeping on is going to falter.
The antidote, of course, is to start off with realistic expectations and realistic optimism. If you are more realistic at the outset, your initation of the weight-loss process might suffer from not enough initial oomph, but because of these more realistic expectations you will be able to keep at the weight-loss process much better.