She has for a few years.

I agree with dadaist. My SO has just been through a wicked divorce that lasted three years! When I met her, she was on anti-depressants and had no natural lube. Over time, she was able to dispense with the meds, and she now gets wet just thinking about sex.

Yeah problems with sex are very common in anti-depressants (inability to orgasm with men AND women, inability to lubricate with women). That's why it made me think of it. But if this doesn't apply to the person in question, then other things need to be considered.

I agree with what everyone's said here, but I have to say that given the circumstances I'd be most likely to go with what Lixy said.

You must remember that at 19, your girlfriend may not have learnt to be as honest about your sex life as she may make out (speaking from experience here) and she may now be worrying about how wet she is more than she's thinking about sex.

I would recommend that you at least try getting her as turned on as possible (rather than just 'turned on enough'). Double the length of time you spend on foreplay and don't penetrate her until she's literally begging for you to do so. Even if it turns out that the dryness has nothing to do with how turned on she is, no woman is going to complain about this sort of treatment.