Cystic acne from a long time. Help me.

It all started when I completed my class 10.In the beginning only a few pimples appeared on the forehead and did not paid any attention to them but when they increased then I used anti bacterial powder.well they get cured after sometime they appeared again but this time on cheecks too.Then I became worried about it started using.Safi syrup as well as clindamycin and nicotinamide gel.They vanished again but not the marks.From that time onway I am periodically having pimples they gets cured and appear again and their marks have destroyed the complection.The natural shine and lustre of skin is gone.It looks like dull and lifeless currently I am using benzyl peroxide soap and clindamycin and nicotinamide gel,And hoping that it will cure the problem.From a long time I am not consuming junk foods and oily foods too but still the problem persists.please help me. Well I am an underweight person have poor appetite ,the skin of my whole body looks dry but face always remains so oily.so help me.