Overestimating? As in you didn't take on a project because you thought it was too big? If so plan to start things--starting isn't proportional to total effort. Or do you mean underestimated as in things took more time than expected? In which case you put too much on your list. Prioritize and remember Warren Buffet's 'two lists' top priorities and other priorities (aka. avoid at all cost) if you really mean to achieve your top priorities.