The majority of my clients have "nice person's disease" ... the desire to please others by saying "yes" ... "sure" ... "absolutely" no matter who is asking or when they're asking or what they're asking. Much of the time, its to their own detriment, and the detriment of their practices and businesses.
In case you need it, let me give you special permission to say "no" (or one of its cousins "not yet" or "how about the 12th of never?").
You are already busy using all 24 hours in your day, so taking on new projects and tasks is a matter of sorting, prioritizing and perhaps discarding. Sorting is the picking and choosing - you have a finite amount of time to execute tasks - you must look with a critical eye at each task prior to committing. Prioritizing is taking the chosen tasks and putting them in order of importance.
You must do these three things when a "new opportunity" for extra work comes your way: (1) separate false urgency from real urgency using your education and experience, (2) set expectations for beginning and completing the work based upon what you already have on your plate and the true urgency of the item, and (3) let go of your need to please the person(s) asking and do what is best for you, your practice/business and your life.
Its okay to delay other's desires for instant gratification and continue to stay focused on the task at hand.
Coach's Note: This is your life. Make it the way you want it. Keep in mind that you are the King or Queen of your domain - you are far from being at the mercy of your boss, partners and/or clients.