Caffiene/Sugar headache....suggestions?

Hi All.....I used to drink Coke quite often and have cut out recently and I am trying to substitute my sugar for my coffee with a sweetner. Problem is I get a nasty headache because my body is so used to the sugar and I am not positive how much sugar I can actually consume during the course of a day. Is there any suggestions out there as to what I can do to prevent these headache?s....they literally stop me in my tracks....thanks

Rob