20" Have you tried knocking certain foods out of your diet? I know everyone's got advice on this and we are all different so what works for one doesn't work for the other.
I reached a similiar position to you, lost weight on a couple of diets but then put it back on. Finally I decided to spend some time figuring a few things out.
Firstly I dropped certain foods out of my diet whilst sticking to a fairly routine eating and drinking pattern. The food that made the most difference for me was bread. If I ate bread I gained weight, full stop.
It took a while but I have now eliminated bread from my diet altogether. Next I figured out how many calories I actually needed to eat on a sedentary day. I kept a food diary for this. Turns out I don't need more than about 1800Kcal.
Then finally I put the whole lot together and began to lose weight, gradually. So over the last two years I have continued to lose weight, slowly but irresistibly, in periods, because you can't do it all the time.
That's what worked for me, you won't be the same, everyone is different but you have to figure out a method that works. Once you're on the downward slope you're metabolism seems to change and it becomes quite difficult to put weight back on, provided you're eating correctly for you.