I'm 59 and have had weight issues most of my life, including weighing over 310 lbs (at 5' 6"). I've finally stabilized my weight after years of 'practice' getting it right.
Over the past 35 years I've lost 160 lbs in a 9-month period; 55 lbs in a 5-month period; 40 lbs in a 3-month period; and, lastly, 65 pounds in a 14-month period. Guess which has been the lasting success?
There is no replacement for time and relearning eating habits. The quickle diets such as the one you suggest will certainly take off weight, but the weight will return. The weight WILL return. And, unfortunately, it will probably return with a few additional pounds due to metabolic issues.
We put on weight 'over time', so why is it we think we can take it off overnight? Mostly because we aren't thinking -- we're reacting. We wake up one day -- or we have an epiphany over a few days -- where we say "I've had it! I'm done! I'm losing this weight! I'm going to do it!" This sounds great and positive. Sounds as though we've finally got our act together, right? Sounds pretty noble, doesn't it?
Wrong. It's the same reaction that causes us to down a pound of chocolate. It's an emotional reaction. In the case of wanting to take weight off, it is probably an overdue emotional reaction, but it is still just that. A reaction.
You must make a decision to lose weight.
That doesn't mean that you have to sit still and/or keep eating chocolate while you're getting in the right frame of mind. At any moment, you can choose to have one bite of chocolate rather than a whole pound, or 1/2 pancake instead of a 6-stack. At any moment you can choose to eat an apple instead of a bag of chips, or choose to have a salad or vegetables rather than mashed potatoes. At any moment, you can decide to walk around the block instead of watching another tv show.
Unfortunately, all of that sounds boring. I know. I've been there a thousand times. But if you make a decision to go slowly and carefully, to retrain yourself, you will gain life-long rewards.
Find a legitimate eating plan such as Weight Watchers. Look up the Diabetic Association's or the American Heart Association's guidelines for eating. Lose 1 lb a week. Yes, I know! That will mean it will take 21 weeks to lose those 21 pounds. But you will have really lost them. During those 21 weeks you will have learned a process. You will have learned about how to manage your cravings and emotional eating issues. You will be a smart and educated eater ... a smart, THIN, educated eater, and you'll have created building blocks that help you make good choices your entire life.
Honest.
Good Luck.