It sounds like you're not eating enough and you're getting the majority of your calories (empty calories) from the soda. 1,000 calories isn't enough for a person under 120 pounds...... When you lose weight, your daily caloric intake should taper down as you get into a lower weight 'bracket.' You'll have nowhere to go if you start with 1,000 calories. Here's a website where you can calculate how many calories you need per day.
http://www.icsdna.com/urwhatueat/tool4.html
As for the coke, I'm not better with my diet cherry pepsi habit. My husband hates the taste of diet soda as well, but he will drink diet wild cherry pepsi. You may want to give that a try. You may want to try getting an iced-tea maker and using Splenda. (just to get your caffeine fix w/out all of the calories.)