Hi Mandy, I agree with Harassed Mother! If you are going to cave make sure its something that won't take you out of ketosis. In my desperation I had Quorn pieces and 80g of steamed green beans. You can also have a tin of tuna (in water, not oil or brine) and pretty much a whole bag of salad leaves which will make you feel like like you are having a big meal. Just don't use salad dressing or oil. The next day you might feel guilty, or you can just turn those negative feelings into positive ones by getting back on the diet and feeling stronger. After I had cheated the first time I started to pick at things like crackers and crumpets, and then caved in one very bad day to eat crisps and chocolate. During those weeks I still lost 3lbs a week and I thought, oh thats good - I still lost. But now I just think I might have lost, but it could have been double the lbs if I hadn't cheated at all. When you go out of ketosis you undo all the hard work you have done since your last weigh-in. Its such a tough diet, so I think you need to do what works for you psychologically. Its better to fall off the wagon and have one carb-free meal and then get back on it than to keep going and being unhappy to the point where you just give up totally.
I have found lots of support on here, as well as some very good tips - such as freezing tetra packs to feel like you're having a dessert, and drinking sparkling water to feel more full.
Xx