Miriam,
Sounds like you have had a variety of good advice. Andrew is absolutely right in that you have to figure out what works for you and make it a lifestyle and not a diet. I'm still struggling with that one.
I've also heard it said if you don't get enough sleep it can be hard to lose weight, but don't ask me to explain the chemistry behind it.
For many people blood glucose levels effect the body in how it gains or looses weight. The idea of more protein is to lower blood glucose levels. Along with that there is the glycemic index and load of foods that effect blood glucose levels. "High-glycemic foods like rice cakes, bread, and potatoes stress the body's insulin system and probably are chief culprits in obesity." Very low glycemic foods are lettuce, celery, broccoli, squash, tomatoes and other foods of that sort. Fruits would be strawberries and raspberries. Proteins and fats are very low glycemic as well. Foods range from very low on up to high and there are lists that explain what is what. For a while I was really trying to pay attention to the glycemnic load of food and learn about it. I lost a little weight without really trying. I forget what put me off course, but it is time to get back on.
This is an interesting website that talks about it.
http://www.mendosa.com/gi.htm
I would say of course calories pay a roll as well as how much exercise one gets.
One of my problems is that it takes a lot of time to pay attention to everything I eat and to retrain eating habits.
Now if I were rich I could just hire someone to prepare all the healthy meals and create portions where I didn't feel like I were starving, that would make losing weight easier.