Everybody is unique - you may be fat because of fast food but crap never goes into my mouth and I exercise a lot (a lot of crap may come out of it).
This last month I've even gone without my 1 bottle of Newcastle Brown once a week - desperate times.
I don't mean to be rude, I really don't, but you've mentioned(I might be thinking of DMpoole though, sorry if thats the case) that you eat well but are overweight. The real question is that is now, how did you eat when you got fat essentially.
There is a world of difference between eating badly, getting to a certain weight, then eating healthily and finding it very hard to lose weight, than being healthy/around the right weight for your body type, eating healthily and not getting fat.
One of the most important factors in hormones and being fat is, that you produce different levels of hormones depending on how fat you are.
A fatter person will produce a lot more hormones that make it harder to loss weight, than a thin person so its a vicious circle, once you gain weight, you are in a much harder "hormone fight" with your body than a thinner person is.
Genetics plays a factor, body composition at any given time generally speaking plays a MUCH bigger factor.
Some people genuinely put on weight through no fault of their own, or not directly. Severe depression can cause both weight loss and weight gain, depends on the person, an actual thyroid problem(or many other medical problems) can cause weight gain and many factors combine to make a bad combination. A depressed person that has been ignored by doctors who also developes a thyroid problem might not go to be treated by a doctor as they gain weight and get to a point that hormone production, depression all make losing weight almost impossible.
A thin healthy happy person might detect weight gain, feel unwell, see a doctor who catches a thyroid problem before it results in significant weight gain, another doctor might tell that person to eat less, blame the patient and send them home.
However, a LOT of people in this country get fat through eating incredibly unhealthily, THEN start eating well, THEN complain its clearly not their fault as they are eating healthily but can't lose weight.