Don't believe the myth that all a woman needs is tons of foreplay and she'll be wet in no time. Nor that your size intimidates her to utter dryness. That isn't always the case. Sometimes it's medical or psychological too!
Case in point: That 'nervousness' can be a medical condition that exacerbates the situation. My wife has had a general fear of sex for years, a situation prompted by sexual offenses brought upon her before I met her. That fear of violation as well as a fear of discomfort from my size led to many unsuccessful nights, no matter how long the foreplay. Sometimes it is a 'mental blockage' that makes it almost impossible to self-lubricate.
My wife was diagnosed with secondary vaginismus - which is an involuntary tightening of the vagina during attempted sexual intercourse. The dryness was a secondary result of her vaginismus, because she consciously feared the pain and unsuccessful sexual attempts. A sort of 'performance anxiety' on a woman's part.
Some:
1. counseling from a good therapist,
2. vaginal dilators to 'teach her vagina' how to accept, enjoy, not feel ashamed, painful or 'exposed' with a penis on her own terms in the privacy of the bedroom while alone and
3. some conjugated estrogen creams to increase her self lubrication alleviated it all after some time.
Be patient - help her through things, ask how YOU can help and Good Luck!