The only compelling reason that will make you stop surfing the net even when you are supposed to be working is when your boss takes notice of your performance, and he takes drastic measures so you could act on it.
And believe me, you would rather deliver an exemplary performance in the office rather than be known as someone who does nothing but surf the net all day.
It really makes it hard for you to act on it now because there are no mitigating circumstances that will make you stop it. Another thing is the fact that you continue to have access to it, therefore, that little devil in you will still tell you to just go surf.
Initially, you can begin curbing this behaviour by putting priorities in your work, and their corresponding schedules for completion. You can gradually work on easy tasks, slowly managing the complex ones until they are completed.
And after you have successfully met all the timelines for your tasks, you can then consider surfing the net as your reward. This way, you will be able to focus on your job, the very reason why you are there in the first place, consequently alloting time for some relaxation when you dont have much to do.
Your boss will be pleased with your performance, and you will be happy with the way you are doing, leaving your internet-surfing habit a secondary matter to distract your mind off of work.