I'm 5'7"/5'8" and am just over 60kg, I think (ha, I always said I'd never talk about my weight on the internet but here I am!), and I don't consider myself to be overweight. Not underweight, perhaps a bit above average, I don't know...but not fat. I know I could be skinnier, but I think I look all right so it doesn't bother me too much.
I calculated your BMI and it puts you at a "normal weight", so please, don't worry about it! If your weight is really bothering you then do something about it the healthy way - a bit of exercise, eat more fruit and vegetables, cut out things like chocolate and crisps - but if I were you I wouldn't bother with diets. Even if they do work for a bit, you won't be able to keep it up and when you end up putting the weight back on you'll feel worse about yourself.