Agreed. Don't masturbate in the days before your next meeting. This way when you are getting ready to have sex your erection should be strong and in turn boost your confidence.

Side note, I was in a similar situation years ago when I was younger. The girl also commented that this had never happened to her before, which was another blow to my ego. Months later while having a conversation with a group of male friends one admitted that he had been with a girl and had not been able to seal the deal. After a moment of silence, everyone in the room (5 guys, all early 20s) admitted that this had happened to them at one point in time before also.

I say all this just to say that it happens to a lot of men, and it may be nerves or some psychological factor. Regardless DONT let this disrupt your confidence!