Lack Motivation

Nnoo19

Im 19 years old and in the last few years I have went from 12st 4 pounds to 7st 10.
I have maintained this weight for the past 3 months. But I am at the point in my life where I missed out on candies and cakes and general junkfood that the people around me and who I live with have a lot and I want to be part of it.
I am having difficulty with portion sizes right now. I make a small meal and i find I am still hungry after it. I am also craving a lot of fattening foods and its getting me worried.
I find when i treat myself i get very paranoid and upset.
I have also lost all motivation for the gym, i am at a loss in what to do