I like my GF, have sexual issues
I'm a 24 year old male with one semi-serious heterosexual relationship under my belt. I had not been actively seeking another relationship, believing that most women I would meet would simply not meet my expectations, and that I would eventually by chance meet someone appealing. I recently met someone (21 years old) with whom I would like to continue a possibly long-term relationship. We genuinely like each other; she claims to like me enough that she wants to lose her virginity to me. I'm willing to do it, even though the emotional intimacy means a whole lot more than the actual sex. The problem however, is that anytime we get close I just can't stimulate myself enough to stay hard long enough. I can only stay aroused by having homosexual thoughts.
This is very disturbing to me, because I would really like to have this relationship work, on all fronts. She suspected in the past that I might not be completely straight, and begged me and cried to me (as if it is something I could control) to convince her that I am not homosexual.
My main question is, is it possible for me to condition my brain, perhaps by masturbating alone, to be sexually attracted to her, enough to fulfill her desires? Presently, I have too many homosexual fantasies while masturbating.
Thanks