How do you recover from failing to get hard?

Ok so, I'm 16 and have never had sex before, I was about to have sex with my gf for the very first time. I was perfectly erect before, all of our clothes were already off, I put the condom on, however after a 10 second or so pause, I went soft. I had to tell her and we both tried everything we could for 20 minutes to sort things out. No luck. She found it funny and was not dissappointed with me in any way. However my confidence has taken a major battering, I have been great at everything else sexually related with her but this went very badly, how do I ensure that next time I will end up going great? Without the use of viagra.