Disobeyed My Rules

I made a rule for myself that I wouldn't snack after dinner because it's bad for my health. But tonight, I just couldn't resist the chocolate cake left over from last night's dessert. I knew I shouldn't have eaten it, but I did. Now I feel terrible about breaking my own rules. It's like a little kid getting caught with their hand in the cookie jar. I try to be disciplined and follow through on my promises to myself, so this feels really wrong.