I know how difficult this problem must be for you as I suffered with the same issue in my teenage years and early twenties. I tried every pharmaceutical drug and alternative remedy going, none worked.
What lead to my acne clearing up completely was a total change in diet (I took out all meats, dairy products, dried food, alcohol, oil and replaced it with more fruit, veges and fibre). I also took up meditation to reduce my stress levels, the acne on my back and shoulders cleared up completely after approx 6 months and has never returned.
As well as clearing up the acne, my general health improved immensely. This may sound like a big change - but it was definitely worth it.