the only thing holding you back from success
You are a person. This means that you have the ability to do whatever you want whenever you want within reason. You can make your own choices and choose how you react to certain situations. In life, people strive for success in whatever field they enjoy.
In order to achieve success in most lives, people must develop the ability to say no and walk away from situations that will inhibit you from getting to the goal of 'success'. In every person's life this means staying away from drugs and alcohol as a minor. Failure to abstain from doing drugs or drinking will likely result in your goal of 'success' crumbling at your feet.
The only things keeping you from doing good things, and staying away from the bad things is you. Since you have the power to make your own decisions, you are what you must overcome in order to be successful. You are the driving force behind what will make you successful, but you are also a potential flaw that may crush your chances of achieving your goal.
The Finish Line
Once you overcome the only obstacle in your path, you, you can do whatever you want, including saying no to drugs and reaching your goals.