help!

I'm trying to lose a few pounds before a family wedding in the spring. I have cut back on snacks and sugary things and been very resolute but at friends for dinner last night fell completely off the wagon and ate cheesecake and chocolate mousse. I'm so annoyed with myself as I have worked so hard at it for the last two weeks and am the sort of person who is better leaving these things out completely because if I have a bit I will invariably have more. Also now having had this I want more which is exactly the reason I stopped myself in the first place. It is really important to me to feel good at the wedding plus I really do want to lose the weight more than anything. How can I conjure up some willpower?