Old Way Of Thinking/Old Excuse: "It's a special occasion, so I am going to eat all I can."
New Way Of Thinking: "Special doesn't/shouldn't mean stuffed."
There is no need to pass up a piece of your own birthday cake, but surely you can have a small slice rather than a huge one. People tend to have more fun when they feel 'satisfied', not stuffed.
Think about WHAT you are eating while you are eating - do I need to eat (all of) this? Or, this tastes great, so glad I am having a few bites, now that's enough!