All the input has been accurate thus far. I have been able to reach orgasm from just kissing yet there were times when no matter what my lover was doing, there was NO HOMERUN and it didnt have anything to do with his skill or lack thereof but was because I simply wasnt that into him OR I had an issue within myself which prevented me from letting go. <----these last two scenarios were definitely the exception and not the rule, but still the cases existed nonetheless.
Now as your girl has such limited experience her not being able may be a combination of any of these issues: not being comfortable with herself (that means it is ALL HER and not you at all but that doesn't mean that you cant help), not being comfortable with some aspect of your relationship (not saying anything bad about you, just saying that she may not be able to effectively communicate her desires to you so in effect, this issue of hers makes it your issue by default as you are her lover).
Loosening her up with a bit of drink and dirty talk may help. Send her sexy texts or emails. If dirty/sexy works then do that, however, if that is a turn off then do romantic - I love you baby type stuff.
The vibrator may be your best help. Go shopping for one together and encourage her to play with it alone if she is self-conscious or play together as this may heighten the erotisicm of the experience.
As for going down or intercourse, regardless of how long you can perform if what you are doing does not do it FOR HER specifically then your staying power is moot. She must learn to effectively communicate to you what feels good, what feels bad and what REALLY feels tremendous.
Try not to make her feel like "there is something wrong with her" but instead take the - I am soooooooooooooo into you baby I just want to please you till there is no tomorrow. YOUR PLEASURE GIVES ME PLEASURE.... Just play.... Think about how crazy YOU will be when you do get her off but YOU dont get off. Just make it all about her and be done. THIS WILL BLOW HER MIND. Then when you guys do finally achieve mutual orgasm or she gets off several times (course not talking about all in the same session necessarily) and you havent been able to get off - then finally you do... Holy Mother of God!... that will be ONE INTENSE experience.
Keep it light and fun and TALK TALK TALK.
Please remember that your most powerful sex organ is between your ears - NOT your legs!