I have the same problem, especially with the internet. I don't waste a lot of time, but it's those thirty minutes every day that I could be using on things more important to me.
I'm a high school student - when my parents took my laptop (my only computer) away from me for a few weeks, I noticed I had a lot more time to do things I considered to be more important.
My suggestion is to set rigid guidelines for when to be on your computer. I give myself one hour (from three to four pm) every day to use as free time (which I try to spend responsibly - cleaning my room, checking my email, etc.) - in your case, talking to your girlfriend or doing stuff you enjoy - and then use Steve's timeboxing techniques to cover the rest - (schedule yourself x amount of time to accomplish something, and stop at the end of the time, even if you weren't complete)
I think this is a pretty common problem, but it's very important to improve it.