Like many people on this thread and elsewhere on GN I'm a life-long dieter. Usual story - lose weight, gain it back, different diets and so on, over the decades. Genetically I'm a short apple, as is my sister and as was my mother and grandmother, and have hypothyroidism.
I'm currently slim, after hard-core dieting for the past five or so months. It's been such hard bloody work - a few meals out and you gain 3 pounds, then those 3 pounds take weeks to get off.
Now I have two choices, to stay as I am by sticking to a reasonable diet, or letting it creep back up which, as we all know, is so easy to do. Apart from serious mobility issues & those few people who have an underlying medical condition, anyone can lose weight if you stick to the diet! We all know it.
Keeping it off is a different matter.