I had a lot of problems with my diet when I was trying to change it a long time ago; I couldn't stick to a plan for more than a week. I knew that I was able to push myself in other aspects of life, such as hockey and school, yet I couldn't stick with a simple change in diet.
I definitely would say there were a lot of problems with how I was approaching it:
-A had very little knowledge about how to lose sucessfully
-I would often say "I am starting on X day", but the next week, it was "I'm starting on this day," etc. I really had no starting point every time I tried, I was so random.
-As others have said, I would make excuses. I could eat perfectly until suppertime, but as soon as I saw the rest of the family making pizza for supper, that was it - I don't know how I justified it to myself that I should be eating pizza instead of making something simple for myself, like soup and sandwich or a frozen dinner. A lot of times when they made something bad for supper, I would tell myself "tomorrow is a new day, I can just start over."
-I was a horrible late night eater - eating past the point of feeling full, and going to bed with an upset stomach. 2 years ago I was fairly depressed (don't have clinical depression), so I was more or less "feeling down", so I turned to food a lot at night. Even when I wasn't feeling down a year later, I would stuff my face at night, even though I KNEW I didn't need it and didn't really want it. It became a reaction and a habit.