Can you switch to diet or Coke zero just to start to wean yourself off? At that point, the calories will be reduced so it will benefit your diet a bit. Once I moved onto flavored seltzer (non-aspartame) I never looked back. I really only wanted the fizzle. Once I came off of coffee for the most part, I did one regular coffee a day (ie with caffeine) and the rest would be half decaf or full decaf. It's amazing how we can retrain our bodies to "like" what we want it to.
And....the amount of calories I would drink in a 2 liter of Coke/Pepsi/Mountain Dew, etc. a day actually grossed me out so I had to walk away from it.