Well done for cutting something out and losing weight to try and improve your health, this is an admirable goal. However, from your tone I would hazard a guess

Well done for cutting something out and losing weight to try and improve your health, this is an admirable goal. However, from your tone I would hazard a guess that the problem is not your diet coke so much as your desire to consume so much that you are choosing diet coke as an optimal lubricant to enable that. You can't compare a habit like this to smoking imo, which is chemically addictive with a reasonably well understood chemical pathway and requires considerable willpower to stop due to physiological withdrawal symptoms as well as the psychological aspects of addiction. Maybe you have some underlying problem which is causing your behaviour, because most people do not require very much willpower to not eat when it is uncomfortable to do so without 'lubricating it' optimally with some specific drink. Or maybe I am just misunderstanding and overreacting based on how shocking I find your comment...