I think the problem might be on jerking off too much. you can try not to jerk off for a week before you have sex with her.
But i think maybe if she try to give you a blow job and things like that that. it might helps. always try new things and at the main time, give her some pleasure while she is complaining.

i don't know if women can post here. if not, please forgive my indiscretion.

i think your issue is anxiety related. something is bothering you about the experience. maybe you're not that "into" her, or her being younger than you...maybe she's not of legal age?

i don't know, but i definitely get the vibe that something is bugging you out and that's why you aren't performing as you'd like to.

if i'm right, then what you need to do is come to grips with what's bothering you and then you'll be fine. maybe you need a different girl?

one thing that bugs me is that she's complaining about this. if she's the right woman, she won't complain, she'll work with you to make things great.
jasmine