Since you don't have this problem when masturbating, then you know there really isn't anything wrong with you. Maybe you got used to one way of orgasming (from masturbating) and sex with your girlfriend doesn't duplicate those sensations, so now it is hard for you to get off that way. Maybe you're not that turned on by your girlfriend. Maybe now you're just thinking about it too much, setting up a self-fullfilling prophecy.
I wouldn't advise you to just "enjoy" your ability, because obviously neither you or your partner are enjoying it. Intercourse that goes on forever tends to get boring and makes the woman sore. And yeah, on some level it is insulting to one's femininity :-) As a woman I would start thinking that my partner must not be that turned on by me.
So, what can you do? Hmmm. Maybe you could engage in other activities that take you closer to orgasm before actually having intercourse, such as oral sex or a hand job? Think that would help?
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Long Stamina 
Linear Mode
