Do you mean diet as in dieting (ingesting less calories and therefore being hungry) or diet as in what you eat everyday? Actually most people don't achieve more than a few months of dieting (eating less calories).The long term diet interrupts habits more severely.
That's wrong on multiple levels:Your previous level of eating was what caused you to become fat in the first place ...
a) I have never actually been fat in my life, only slightly overweight, but I find the topic fascinating.
b) Often it's not the amount of food that makes you gain weight, it's the change of circumstances. People maintain their weight for years, sometimes decades then circumstances change and -baam!- weight explodes. Reasons include more stress (raises cortisol levels) and lack of (restful) sleep. Most people who become parents can tell a story about that.