From my experience and observations a big reason diets fail is because the dieter has the attitude, "I only have to suffer for a few weeks and after I'll eat a whole pizza." Having this attitude is setting yourself up to fail. You can eat clean and be happy, believe it or not you CAN be happy without donuts and junk food. If cutting all that stuff is too hard, start slow and take one thing out of your diet at a time. That's what I did. I started with sweets because I don't crave sweets so it was easy to stop eating those, then I took out Cheetos because I LOVED Cheetos and then fast food and then cereal. And now all I need to cut out is diet soda which is very difficult, so if you see me drinking a soda, don't follow my example. That's my big goal for 2013.
Remember, it's a lifestyle change and that does happen over night. Commit to eliminating one thing at a time from your diet and it'll be easier to succeed. After 12 weeks of clean eating you'll be surprised how much you love it and how good you feel!!