Hello and welcome to e health forum.
From the description it seems that you have body dysmorphic disorder.
Based on the height and weight, mentioned here, you have a BMI (body mass index) of 19.26, which falls under the normal range of body weight.
Here is a description of BMI for your knowledge:-
BMI less than 18.5 = underweight
BMI 18.5 - 24.9 = normal
BMI over 25 = overweight
BMI over 30 = obese
Inspite of knowing this that you are having a normal weight, feelings or actions to reduce your weight to much lower levels can be counter productive and can cause long term problems.
Being more skinny - does not necessarily mean that you should be underweight. Comparing yourself with other individuals is not the right way. You should have confidence in yourself and your appearance.
You might benefit by consulting a psychiatrist, who can counsel you and provide proper help.
I hope this helps.