The short answer is being slim for most people is a life style not a diet.
If you go on a diet for a couple of weeks or a couple of months and lose some weight, if you then go back to your old bad eating habits you will always regain the weight and possibly some more.
So to have lifelong healthy eating habits you need to address the reasons why you are overeating in the first place. Is it emotional eating? Is it boredom eating? Or is it just a bad habit? The trouble with all these things is they become subconscious behaviours and therefore difficult to change by just making a conscious decision to be healthy.
The fastest way to change subconscious behaviours is with hypnosis because it deals directly with your subconscious mind.
So if you do want to be slim permanently you might want to consider hypnotherapy.
Please note results may vary.