Hi Kirsty, I have just read you are 16 years old. To me this throws a different light on things and I hope you won't find me patronising.
I don't want to pry into your family background but I'm assuming it must be your mother/parents that buy the food at home. You don't have to say but are they overweight? It may be easier if you and your family tackle weightloss together.
At any age it is difficult to cope with the emotions of being overweight but at 16 you have a lot of hormonal stuff going on not to mention as well as the normal teenage stuff involving boys, school/college etc so I appreciate why you are feeling so low about yourself and your weight.
You say you have tried so many times before and failed although you did fantastic when you lost 10lb a few weeks ago. That was a huge achievement so it shows you can do it. So the problem seems to be that you can do it for awhile but then find it hard to keep it up.
At your age I imagine that a lot of your social life involves meeting up in macdonalds and places like that. I'm an old bat and mother of two so don't have that problem but as my son is 15 I can see how most teenagers meet up in macdonalds and seem to spend a lot of time eating rubbish! Yes it's unfair that some of your friends eat all this stuff and don't put on weight but you have and do.
Again, I don't want to pry but you need to tell someone in your family be it your mother, grandmother, father, whoever, how unhappy you are and that you need help. You are after all still a child (sorry I REALLY don't want to sound patronizing) and I don't think your losing weight is something you can do on your own at the moment.
Aside from what you and all your friends eat, what are you eating at home as that is something your family can help you with. I urge my children to eat up because that is something that us mums do so do you feel guilty if you don't eat your tea?
I also think you could benefit from seeing your GP especially if your family are overweight and taking your mum or whoever is your main parent along would show them that your health is at risk.
I'm sorry, I don't know you personally so don't know what is going on in your life but I can see that you are an unhappy girl/young lady and that this should be a glorious time in your life. I feel that you are in a lot of pain and anguish and know that appearances really matter to young people and that they put huge pressure on themselves to fit in with their friends.
Kirsty, please don't despair, you can lose weight but you need some help from your family/caring adult and possibly your GP. x