How about thinking of WHY you need to lose 2stone by Dec 21st? List all of the reasons you want to lose the weight, and keep reminding yourelf of them constantly so that when you are tempted to nibble, they pop into your head.
Split your packs, so you can space them out over the day, and keep a bottle of water with you all the time so you can sip on that instead.
BUT if you are finding you really cant manage SS, dont beat yourself up about it, maybe try 790, which I hav heard has similar losses?!
Good luck!!