If it's something you truly can not control, that you just can't stop yourself at a very small portion, then I feel it's better to not start with it at all. Food is pleasurable. We take one bite, it tastes good, so naturally - WE WANT MORE. If you don't take that first bite, that you won't want more. You won't know what you're missing so to speak.
When the day is over and you've done without it, you will be so pleased with yourself. The more times you say no to something, the easier it gets the next time and then the next time.
You very rarely hear someone say, "Gosh, I'm so glad I blew my diet on all that cake. It was SUCH a good decision to eat all those excess calories."
Good luck. I'll be thinking about you.