We often say "yes" to others because we want to please them. But when eventually we can't continue, we let them down and we feel guilty. Both parties suffer. Recognize that a desire to please often prevents us from saying “no”.
You have a right to say “no”. Remember that others may take you for granted and even lose respect for you if you don't.
Ask for help in deciding where the new task should fall on the list of priorities; ask: "What would you like to give up in order for me to do this?"