I'm going to ask my roommate not to invite his girlfriend over.
Why don't you want his girlfriend to come over?
I don't want her to be here today.
That's understandable if he's bringing her over 24/7. You need to sit down with your roommate and make a contract on how often guests are allowed. If you don't do anything it will make you unhappy.
Dammit. This always does happen with me.
Don't worry bro, I know it's frustrating and annoying. Like I said, just have a discussion with your roommate and see if you can reach common ground. This issue is quite common so it's okay to bring it up and I am sure your roommate is expecting it as well, but if you don't say anything he would just pretend your cool with it.