Sexual health and pleasure are very personal. Open, informative, and accurate communication regarding sexual issues and body image between you and your girlfriend are essential to mutual pleasure. Couples who are open and honest about their sexual preferences and feelings are more likely to avoid some sexual dysfunction. One partner should, ideally, be able to communicate desires and preferences to the other partner.

Review all medications, both prescription and over-the-counter, for possible side effects that relate to sexual dysfunction. Avoiding drug and alcohol abuse will also help prevent sexual dysfunction. You might also suggest that your girlfriend learn "Kegel exercises"....Kegel exercises strengthen the muscles of the pelvic floor and can lead to a tighter vaginal opening. More info here:

http://www.nlm.nih.gov/medlineplus/ency/ar ticle/003975.htm

i had this problem with my current boyfriend 2 years ago. in the beginning i couldn't feel him inside me, and now he fits snug as a bug and our sex life is dynamite.
he told me to do kegel exercises. this is when a women squeeze her muscles as if she is stopping her urine flow. she has to do that on a daily basis.
another thing you can do is let her ride you.
i think it took about a year for my body to adjust to his size.
try to talk to her, you have to choice your words b/c she can be very offended.
most guys dont give enough foreplay. can you try to have 20 minutes of foreplay before rushing into it. my boyfriend who reads alot says that its always some kind of sexual technique. so search for websites about your problems, that might give you good ideas