Accepting the right mentality

Overweight people always think they don't eat a lot / Underweight people always think they eat loads

For the most part this is purely in the mind. Yes different people do have different metabolisms, but on the whole the variance isn't that great. I've known underweight people that swear they cannot put on weight and they eat loads. When I've monitored them they usually have one big meal a day and a few small snacks, not even 3 good meals. I've known overweight people that usually try to eat less, but eating a lot to them isn't one big meal in the day, it's several big meals, plus snacks, plus chocolate, ice cream and other high fat foods.

You got overweight and unhealthy by over eating.
It doesn't matter if you eat the right daily amount now, you got unhealthy by over eating. Eating the right daily amount now only means you shouldn't be putting on more weight. You will not lose any weight by eating the right daily amount. Over eating can even be following the GDA. If you aren't doing any exercise, then the GDA will likely be more calories than you are burning.
You have to under eat to lose weight.
The only way to get your body to burn fat is to under eat. Eating your GDA for fat and carbs will just keep you where you are.
You don't have to eat when you feel hungry.
When an animal feels hungry it eats. When it feels tired it sleeps. When it needs the toilet it goes. We are animals, but thanks to an over-developed brain we can ignore our animal instinct and Choose to do something else.
Tell yourself something enough times and you'll do it.
This doesn't just apply to weight loss, but... If you are sat there telling yourself "I'm really hungry, I really want to eat..." then you will end up eating. If you keep telling yourself "I don't need to eat, I can wait..." then you won't.
The BMI doesn't lie
The question of whether you are overweight isn't opinion. It's a medical fact. Check your height and weight against the BMI Chart. People are often quick to say that the BMI chart doesn't take into account muscle, which is true, BUT you have to be a body builder with a lot of muscle for it to be inaccurate. I can bench 120kg, and I'm still in the 'normal' range for BMI.
True friends are honest and supportive
Some people find it hard to admit they are overweight and unhealthy, and don't take kindly to it being pointed out. Good friends will talk to you about it and offer to help and support you to lose weight. Don't bite their head off, it's not their fault you got to an unhealthy weight. Other friends will try to tell you what you want to hear, saying your weight is fine how it is. The fact is that if you are overweight (or underweight for that matter) then there are lots of health concerns. If they really cared they'd be honest and support you to lose weight.
Don't take it out on others
Just like someone quitting smoking, for some people the changes in your body will make you want to be nasty and take it out on others around you. DON'T! People shouldn't have to tip toe around you. This is your issue that you created yourself and now have to deal with yourself. The world is not against you and those well meaning friends trying to help you aren't your enemy. It's not everyone else's fault you feel like this, it's yours. And no, letting yourself get wound up doesn't mean you can give up and start eating cakes. These feelings will go away as you get used to dieting.
Take responsibility, take control
Above all, you need to take responsibility for your weight. It's very easy to blame someone or something else. Whether that's advertising on TV, or a relationship that is making you miserable. If it's not your fault, then you are powerless to do anything about it. Only in taking ownership of the problem will you be empowered to change. You've been the one picking up the food, you've been the one putting it in your mouth and eating it, no one else. Take responsibility, take control, and give yourself the healthy happy life you deserve!

Read more about the diet plan.