19, can't ejaculate with girlfriend. Why?!

19, can't ejaculate with girlfriend. Why?

Up until a few weeks ago I was a virgin. But i'm in a serious relationship now and whenever we have any form of intercourse, I cannot seem to ejaculate. There is pre-ejaculate present, but I don't actually orgasm or ejaculate. 2 hours of straight intercourse hasn't done the trick so I doubt if duration is the issue. Help!
Thank you

First of all, are you using a condom? I know from experience if you are used to wearing one they can cause a loss of sensation and thus not allow you to orgasm, especially if you have been used to masturbating all this time.
You also may be overly anxious about having sex and might be putting too much mental anxiety on yourself to orgasm. This may especially be true if you, indeed, aren't using protection and you could be worried about unwanted pregnancy or diseases, etc.
You may also not be getting enough foreplay. You just may not be aroused enough prior to intercourse.
How often do you masturbate? It's a possibility that you have very little semen left at the time of intercourse and it takes just too long to build up enough to orgasm and ejaculate.
One last possibility is that you are not receiving enough stimulation from your girlfriend's vagina. It could be she is getting so aroused that she is producing excessive amounts of vaginal lubrication thus not allowing you to get enough friction onto your penis and not arousing you enough to get to orgasm.
Your situation could be caused by other physical problems like any medication you are on, lack of sleep, smoking, illicit drugs, stress, etc.