Can I put a big ol be careful warning about bending the rules and I'll do it speaking from my own experience.
I've been in OA, done Fit For Life, did CD last year and Scarsdale at one point too. Any single one of these diets could have worked but it was me who had to bend the rules, broke them and hence regained any weight I'd lost.
With all the money I am paying out for CD (for the second time), this time I am absolutely going to stick to it. I've realised in CD we learn how to stop eating and in CD we learn how to start. It's about a big word for me called Discipline and really thinking the consequences through.
I did fine on SS+ (my first attempt on CD) and didn't want to break the rules once. However then I started moving up the steps and decided I knew better. Indeed it started with 'just one thing' off the plan, I justified funnily enough by saying I felt dizzy and unfortunately I didn't stop until I regained all my weight.
Now not for a second am I suggesting you are going to do the same moggy but there is a risk that just one leads to the next and then..... Especially with diet coke as the caffiene turns our stomachs acidic and that *can* lead to more cravings.
If the above helps, try to take the time to think about why you feel you 'need' to have these things. Could you have saved half of a pack and had it later or even walked for a little less time? Were you thinking you 'deserved' the coke because you'd been good? The wise KD always reminds me that food isn't a reward
Please forgive me if this sounds harsh as it's not my intention. I just want to share what happened to me when I broke the rules. Of course you need to find what works best for you and I wish you the best with whatever you choose
