Despite your prediction of how much you are going to spend each week, do you find that you are spending more than you predicted? Do you do the same with food: predict you will eat less, but then you eat more?
There seems to be an inherent optimistic bias in this kind of self-prediction. What is missing is the corrective; that is, being able to draw on the result of past experience.
Standing in the way of taking these past experiences into consideration is how up-close you are to your prediction. The predictions you make about the upcoming week will probably be based on how you will implement and pursue your goal this week. If you were to predict whether or not you will eat less one-month from now, your purview will in all likelihood be broader. Odds are that you will base your judgment on a more complete array of your past behavior.
So don’t be disappointed in yourself if you predict each week and each week you can’t live up to your prediction. Take the longer view and make your predictions more accurate.