Excellent question, and one we actually get quite a bit here at JA. The level of narcissism among society as a whole seems to be growing, so I would bet that you are spot-on in your armchair assessment of your girlfriend. Here are a few things to keep in mind:
(1) To begin, I would ask some very serious questions of myself regarding what has driven me into a relationship with a narcissist. What needs/feelings have you been taught to deny such that you are capable of giving up your own "self" in favor of hers? And I am not saying you should or should not be with her - rather, I am saying that there are personality variables within your constellation that make it possible to tolerate some of the actions/quirks of a narcissist... and these variables may create low spots for you (depression, anxiety, etc).
(2) Okay, but more to your question directly: The way that I have found most effective in "telling a narcissist that they are one" (yikes!) is to instead talk about their parents. Often, a true narcissist will come from a parent or parents who mirror these exact tendencies - BUT - when the PARENT does them, it makes the child crazy. So, your girlfriend can easily spot these issues in her parents because they caused her pain. Therefore, a less direct approach would be to ask your girlfriend if she sees some of these same qualities in herself once in awhile. Afterall, it is pretty normal for a child to begin to reflect personality variables from her parents.
In doing it this way, she may be able to connect some dots in a manner that is less threatening than direct feedback (which is typically intolerable to a narcissist).
(3) If the relationship ever comes to a point that you begin to feel stronger at maintaining your own boundaries and she is "punishing you" for being wrong/cold/distant (whatever), you can also try this approach: Tell her that YOU are the one entering therapy, but would like for her to join you, at least for the first session or two, so that you can be certain to not forget anything, or so that the therapist can get her point of view when assessing what is wrong with YOU. In other words, you are making darn sure to not directly accuse her of being culpable in any way for the difficulties in the relationship. Appeal to the narcissism (the best way to begin to move it is to acknowledge and "join" with it - work it from the inside) and use it to leverage her into the therapy session.
(4) If you haven't already done this, there is a great book called "Loving the Self-absorbed" by Nina Brown. I recommend reading it, as it does not tell you to run far, far away (like you mentioned many of your friends are telling you). Rather, it lays out a plan to salvage the relationship. I think you would get a lot out of it.
Best of luck to you. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-