You're young, inexperienced and your hormones are going crazy. After a while they may start to become more and more infrequent. Your girl isn't going to leave you because you get an erection all the time.
Alot of the time when i'm on a bus I get an erection and the majority of the time i'm not thinking about anything sexual. I have to put my bag infront of me so no-one can see I have a boner. Seriously dude, don't worry about it, it's completely normal.