I always felt the same way... said I had "no will power", but like others said, that is a defeatist attitude, and I didn't like thinking of myself that way (because I am not someone that gives up)...
It's funny that dieting has become something tied to morals. I read a book called Intuitive Eating that helped me with this morality that we tie to eating quite a bit. The book talked about how we as a society have distilled down dieting to black or white, setting impossible standards for ourselves. For example, if you adhere PERFECTLY to your diet for the day, that is considered success, but if you so much as have a single bite of chocolate, you fail (gasp)... Once you have "failed", guilt, shame, etc. set it and many of us just say "screw it" and eat as much of whatever we want after this so-called failure. This is the so-called black and white.
The fact is, eating is very much more gray, and not at all black and white. We get cravings for foods, and by trying to circumvent these cravings, we actually make staying on our diet much harder. For example, say you feel like something sweet, but you think you "shouldn't" eat it, because it will result in diet failure. So instead, you get some celery and carrots out of the fridge... you eat this, but you are not "satisfied" because you were looking for something different. So next you eat some low-fat crackers, then some fat-free jello...etc. and just get more frustrated because you are not actually getting what you are searching for.
The book goes on to tell you to really sit down and think about what you want to eat before you eat anything... really think..."is this what I want to eat"... many times, in our diet-mode, it turns out we are eating crap we don't even freakin' like! Once you figure out what you want to eat, as you are eating it, savor the flavors, savor the texture, etc. and really enjoy the hell out of the food that you were searching for to begin with. The book says : Don't eat if you don't like it, and if you love it, savor it"... and finally, tell yourself as you are eating... "I can have more of this food whenever I want"... because it's absolutely true...this small statement alone was the most vindicating thing I have read/ learned about dieting in a long long time. I was so frustrated with deiting/ eating healthy, I was driving myself crazy.. this really lifted a weight off my shoulders and allowed me to enjoy food again.
Another example, If there is a pastry in front of you.. as a dieter, we think... "no, pastry bad because it's fattening"... so we take a bite... and it's good... then we take another, then we think... holy crap, I am on my diet, so I better eat this all RIGHT NOW as fast as I can, because I will never eat one again on my diet. We go into crazy mode sometimes, and scarf down a lot of one thing... but if we slow down and think... man, I can come here and buy a pastry whenever I want, we can slow down, and enjoy these types of foods. It's funny, because many times now I find myself very happy eating a small amount of something that I am really enjoying.
I hope you can stop thinking of yourself as "failing" when you have some chocolate, or sweets, because that could not be farther from the truth. As a human, you have cravings, and they are 100% natural. Good luck, sorry for rambling!