Fareah,
The description of your GF clitoral orgasm only is very common and really nothing to worry about. Google it.
My better half can cum with vaginal penetration some times, but not often. When she does I still must work the magic bottom to get her there. It is just the way she is and every woman is different. They also change over time in many ways, like us only different in their own way.
Keep in mind I am much smaller than you so this technique may not work if you cannot put all of you dick into her and press your pubic area/fat pad against her clitoris area. I can do this with my woman because she can take all of me, balls deep. :)
Get her all worked up, FOREPLAY!!! When she can take all of you gently push deep into her and press your pubic area against her clitoral area. You may need to try different positions to get this to work. Once you are there push gently using small, rhythmic strokes against her clitoris. Concentrate on stimulating the clitoris, small gentle rhythm, move your hips around, move up and down, etc. Your dick is inside her, but you are working her clitoral area. It takes time and patience, and it still may not work, but it is worth a try. It may take several tries.
There are other threads on this site that describe the same technique.
Take care and good luck,
P.S. If you can and do try the technique above drop a line on this thread and let us know if it works for you and your GF.