I'm going back on my word to dorm with this girl for our freshman year at college
Why did something happen to change your mind?
I keep thinking about the possible consequences of sharing a dorm.
If you made a promise, you should follow through with it. It can be very upsetting when people go back on their promises.
I was just joking.
That didn't sound like a joke to me, I think you're overwhelmed by the situation and still feeling worried about sharing the dorm. You should have a talk with a girl to express your doubts. The two of you can work together to make them situation more comfortable for you.