You don't have to tell your boss, because he shouldn't treat you no different. You're completely functional and you got promoted, but if you feel you can't handle the stress, don't accept the promotion. Many people decline promotions (Yes it's true) because of the added days, pay, and most significant: pressure and stress they feel they can't handle, you don't want to be miserable. It is totally understandable, maybe you need to spend more time with your family.