The key is simple. I lost 20 kilos over the last couple of years by doing this one simple thing: Don't eat crap food.
Salt, sugar, fat. Your body loves them, you're genetically programmed to lust after them. But if you get off the treadmill, cut them out completely, after a while you lose the addiction. Exercise helps - doesn't have to be marathons - it re-programs how the body gains nourishment - I imagine it as the CPU gradually eroding the program that identifies and uses "quick fix" sugars fats and starches, and being forced to evolve to use the slow release energy from its own system.
Nutritionists? Forget 'em. Low amounts of processed food - none is best really - candy very rarely, same for fries etc.
Basically, if you eat something and get a dopamine reward, it probably isn't what you need to stay healthy. Oats with water for breakfast, wholegrain brown rice, that kind of thing. You eat it and just carry on as normal.
Bad food also screws up your digestion, "inflames" the system. So you always have stomach ache, which is actually pretty bad.