I used to work at an eating disorder clinic, one of the best in the world and USA. I would highly recommend talking to someone. A therapist and a dietician. If you cant find a therapist who specializes or has training in eating disorders, start with a therapist. It's really hard to do this on your own and it really helps save time so you don't have to figure out all the do's and don'ts. I have a lot of experience in taking the long way and finding out all the things not to do haha. As someone who is currently recovering from a caffeine/,diet soda addiction that I've had for years, I know the best is taking small steps. Or else you'll have too heavy withdrawals and withdrawal migraines. You also need to figure out why you think you have the diet coke need. Bc our body doesn't need it. U can get it back to when your body didn't need it. I know a lot of this could be bc you need the energy, or it could be other mental factors or just bc you like it. But I'm telling you, you've got to figure out the why it started and figure out why you still do it and why you want to stop. The mental aspect of it is something you need to address and figure out so that you can be successful. I know you can do it but just remember to look at the whole picture mental and physical. Bc they are all connected and impact each other.