Changing my diet drasticlly?!?

Changing my diet drasticlly?!?
last night i ate waay to much junk food, and too much food in general.today i ate a half a bagel and creamcheese adn half a grapefruit(normal size breakfast), and i feel fat, sick, and constipated!i would really like to change my eating habbits, try to eat healthier, adn healthier portions!also exercise!but im not sure if its a good idea to take on all of this at once.i know your suppose to work your way up in exercising, and not start off doing 5000crunches your first day.
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