Why is she having pain? It seems that's the main problem here. Is this a partner you've been with for a long time, or a new partner? Some women have real problems having sex. It's not that they don't want to do it, but are so afraid of doing it for various reasons it causes pain. This would usually be in a young inexperienced woman. Or the woman can have a physical problem such as dryness or thyroid problems or other things going on that can make it hurt. There are ways of dealing with this, but you have to know why it's happening to her. A third possibility is that she really doesn't want to do it with you, for whatever reason, but feels compelled to. She might not like sex at all at this point in time. Another possibility is some women's sex organs are a bit misplaced, and if you go in it can hurt if it's not in exactly the sweet spot. In other words, she holds the answer to this, you are going to have to ask her why it's hurting and if it's a medical problem treat it as one. If she's hurting, she's hurting, and that's what you have to figure out. Peace.
Agree with Paxiled on this - if it's pain, it's medical and you need to treat it as such. If she's a virgin, too, it might hurt, and you need to be gentle.
Please stop quarreling with her, though. She is allowed to say no at any time, for any reason. If she says no, you need to just stop. She may need to see a doctor to find the source of her pain, but your role here is to be supportive.
When she says no, stop what you are doing and ask if she is alright. Encourage her to see her doctor. Maybe she isn't ready for sex, so don't pressure her.