I had a bf with that problem when we first hooked up. He told me when we first started dating and he was trying to not be humiliated by an erection he would visualize his crotch being full of ice cubes. Or a train running over his pecker. Or his mother staring at him disgustedly. That sort of thing. Drastic but effective, and after we'd been intimate for a while no longer necessary as the problem solved itself after a while. But anyway yeah--visualization was key.
Could try masturbating as much as possible previous to when you see her, as I'm sure you've figured out. But this isn't a perfect solution, as semi-erection remains for a bit after you do that. And if you're an adolescent (as I'm guessing you must be) you recover very fast and there you go again. Sigh.
Yeah patchy fog what you said doesn't work any other tips? like im gonna be as mature as i can about this but it still gonna sound awkward, but even after i master bate my pecker remains 90% of the time it makes it even worse i just get even harder and it takes for ever to cum and every time i do it takes even longer. so what do i do...i just love sex and always have all my life but now i want a relationship to be about more then just sex and a constant hard on isn't going to make it easy.