You can look at the astrology of weight gain and learn a lot of insight on how hard it is for certain people.
But the bottom line in my experience is that everyone who gains weight doesn't keep a calorie diary. Everyone that I have ever run into has simply no idea how many calories they consume a day.
The strategy for losing weight is simple. You simply have to consume less calories than you expend in everyday living.
Online, there are multiple descriptions of the appropriate number of calories you have to consume per day to lose weight for particular ages and body types. You have to stay on these guidelines EVERY DAY. If your magic number is 1400, then you must make sure that you don't exceed 1500, nor fall below 1300 per day. Exercise will help, but in the long run its simply the calories.
Low Carb diets will make it easier to make the numbers since they tend to reduce appetite.
You also need to drink enough fluid. The fluid cannot be anything that has caffiene in it. Water or herbal teas are recommended, and everyone should consume at least 48 ounces of it per day.
Consume the water, make your calorie number, and walk for 15 - 30 minutes everyday. Unless you have some weird medical condition, you will lose weight.
Again.... 95% of people who complain about weight gain simply have no idea the exact number of calories that they are consuming each and every day. You must keep a written tally every time you eat anything, so at the end of the day, between your last meal and bed time, you can add more if you are too far down.