Try masturbating a few hours before your girlfriend comes over, this might help you last a little longer because you’ve already ejaculated earlier that day. If you notice yourself getting too excited, turn your thoughts to something distant and unsexy, such as homework, or things that need to be done. (ex: I have to go to the bank tomorrow and deposit some cash, have to go buy a book for class, etc) Only dwell on it long enough to give yourself a short break from arousal, maybe 5 to 10 seconds, then refocus your attention on your partner. Avoid thinking of a topic that is going to make you stressed or cause you to lose your arousal entirely. Even try to change positions when your close to ejaculation, just make your girlfriend think you want to change the position that gives you time to slow down a little bit and definitely try focusing on foreplay a little more.