I've liked girls for years, but now even thinking about my girlfriend gets me insanely aroused. Why did this change all of a sudden?
It hasn't changed suddenly. It has merely reached the point where it is difficult not to notice. Your hormone levels have been increasing (unsteadily but increasing) since you've passed puberty. They will peak about the time you reach stage 5 and then remain steady, declining very slowly for the rest of your life.
I don't know exactly where you are in your development, but I suspect that your body has recently kicked into gear producing semen. The strength of the body's desire to ejaculate often catches young men off guard. It isn't proper to be thinking about your girlfriend in a sexual way. You aren't married yet and probably won't be for several years at best. Focusing on what you cannot do will lower your defenses against the temptation to sin. Therefore, as hard as this lesson is going to be, you need to focus on teaching yourself self-control and not letting your body run your life.