Do you ever find yourself taking on more than you can handle, just because someone asked you to? Spreading yourself too thin can be detrimental to your well being. You have to learn when to say "no."
It's more than okay to turn down or walk away from something when your plate is full. Over-extending yourself does not increase productivity, it slows it down. When your fried and frazzled, running around like a headless chicken, you're no good to anyone, not even yourself.
It's never impolite to say no. Bow out gracefully and get on with the business of wrapping up your existing projects. Remember that you're super, but you are not immortal.
Taking care of yourself first is good for all party's involved. You'll always be able to keep your commitments, as long as you stay within the healthy parameters of your own life.