After being in a steady relationship for the second semester of freshman year through this entire summer, I'm thinking of breaking it off with my girlfriend.  

Thing is, I'm really happy with my relationship.  She's a great girl, and I do love her (first girl I have ever said it to, I don't throw the word around).  She can be a bit of a spoiled brat, and sometimes she just doesn't "get" stuff (she's a bit less experienced in the physical aspects of relationships as well as some of the other day-to-day things in relationships) but she is doing much better lately.  Other than those very minor aspects, I know she really cares about me and I, again, am happy with her.

I've just been feeling this "You're in college, you should be drinking, hooking up, and having fun" idea more strongly.  Plus I'm taking 3 more credits than normal, not to mention harder credits, so I'm going to be much busier.  I want to workout more, make more friends, play more sports, and join a club or something, so I don't know how much time I'm going to have for her.

Is it legitimate to break up with someone because of this?  Will I be branded as an asshole?  Beyond that, I know I'm going to be hurting her, but it's for my own happiness.

In actuality you will be doing her a huge favor.  It appears that you think you are superior and you are obviously self centered.  

Sometimes we never place value on what we have until we lose it.  You have to do what you have to do.  You really aren't mature enough to be in a relationship at this time.

Have fun!