I've stayed the same weight since a teenager, but these days I do control the calorie intake, whereas previously I ate twice as much.3 stone in 3 months is too fast. Crash diets and rapid weight loss can wreck your metabolism... plus your body has a weight it likes to return to: homeostasis. The body creates a set point, and when you are under, your body will FIGHT you to get back to that level. Sadly it seems happy to adjust UPWARDS without a fight
Also with rapid weight loss the tendency is to lose muscle... so when you jump off the diet train your higher percentage of fat will make it easier than ever to put weight on.
As we age it just becomes so much harder to change, so we need to be prepared for permanent changes to our lifestyle and habits to have a chance of losing the weight and keeping it off...and to be permanently vigilant about weight change.
Even then, genetics can be against us. It's recently been discovered that there are genetic markers that program people to become, and remain, obese.