loss of erection

i am a 22 year old male sthlete great physical shape and have dated my current girlfriend for 5 months.  during school we had sex often twice a day and it was great.  i felt able to go whenever and barely needed stimulation during forplay and i was always able to get her off at least once during sex.  we have always worn condoms and it was never an issue.  since school has ended we cannot see eachother often since we live far apart. i tend to masturbate twice a day to porn since i am so used to sex. she has visited twice and both times the sex was bad and worse the second time.  she took very long to get aroused and was nearly impossible to get off.  i had trouble maintaining my erection.  when i have suggested trying new positions or tried to get her to stimulate me more during forplay she becomes defensive and is not open to the idea.  i realize some of the problem may be we havent seen eachother and when we get back to school things will work themselves out but i dont want her to leave unsatisied on her next visit.  i am wondering if theres anything i should do as far as taking supplements, using lubes or certain condoms that give more sensation.

[banter]

I bet you have no trouble maintaining an erection when you masturbate - true?  To be blunt (and I read this in a book once and believe it to be true) - your fist is a lot tighter than a woman's vagina.  With masturbating twice a day, you have become used to the more intense physical feeling that masturbation brings.  When you are inside a relatively much less tight vagina, you get much less sensation and therefore you lose your erection.  

With women, using a vibrator amounts to the same issue.  The vibrator brings a much higher intensity of feeling than an erect penis would or could ever bring.  Women can get used to the vibrator so that they then find it difficult to orgasm any other way (such as with regular intercourse).

I am not sure what the solution could be other than to simply cut back on the masturbation (which can be hard to do since the human orgasm is such a pleasurable thing).  I think you on on the right track with foreplay - the longer the better.  Alternately you could both try adding a vibrator to your sex play.  After you each arrive at a particular plateau, switch to regular intercourse.