Like >>2 says, you have other options than simply to break up. Even if you break up, you'll have the same problem during your next relationship (your girlfriend eating up your social life).
So I suggest that you discuss with your girlfriend about the time issue, that you would like more time for meeting people. It's important for your well being, she should accept it.
Now if the real reason is that you are fed up of your relationship, then by all means, break up with her. If you don't find the words to tell, her, write it on a letter to her. After you finish the letter, you may actually know better what to say, and you can phone her, which is the most humane and nice way of going about it. You had a nice relationship, there's no reason not to end it in a nice way. Not all finished relationships must end with angst and drama. And don't worry too much about her, she'll get over you in time, and grow as a person because of it.