Let yourself work under whatever conditions are best for you. Often people with ADD need to be doing several things at once in order to get anything done at all. Do what you're good at. There is no rule that says you can only do what you're bad at. Leave time between engagements to gather your thoughts.
Transitions are difficult for ADD'ers, and mini-breaks can help ease the transition.
Keep a notepad in your car, by your bed, and in your pocketbook or jacket. You never know when a good idea will hit you, or you'll want to remember something else. Set aside some time in every week for just letting go. Recharge your batteries. One guilt-free way to conceptualize it is to call it time to recharge your batteries.