Don't just say you have too much work. Give concrete examples. Explain how you've been assigned several projects on top of your regular workload. And how any one of them would be fine, but cumulatively, you're having trouble keeping balls in the air. Give an estimate of how much time it would take for you to complete each and then the aggregate time. Then explain how it would be impossible to meet all of the deadlines. Tell your boss all the times you've stayed late or worked extra hours to finish, without complaint.