Hey /adv/, need some help here Recently I found the courage to

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Hey /adv/, need some help here
Recently I found the courage to ask out a girl that I've liked for quite some time. We've been going out a little over a month and a half and it's bliss whenever we're together. We constantly touch each other, kiss passionately etc.
Well yesterday she came over (she had done this previously, twice) and we were making out passionately on my bed. However, when it became clear that we were going to have sex , my dick went limp and I couldn't get it back up no matter how hard we tried. She was very understanding and we ended up having fun anyway(fingerbanged and stuff) and she kept reassuring me that this "can happen to anybody". Keep in mind that I'm 19 years old and I have a boner nearly all of the time when I'm with her. It would've been my first time, so I probably just had performance anxiety or some crap. We're still going to be seeing each other and she believes it's no big deal, but I really want to have a sexual relationship with her.
So has this ever happened to you? Do you know any methods to calm down and maintain an erection?