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