I don't know about you, but I never say to myself, "Oh, That's just a small bite, I don't need to have that."
"Oh, Just one sip? I don't need to have that either."
It's ALWAYS "That's just a small bite, I'm definitely having that" and "I will just have one small sip".
Rather than always rationalizing "It's just one bite, I will have that", balance it out 50% of the time with "it's just one bite... i'm not having it".
See what happens over time.