My opinion - she's right, you'll learn to like it.
What are you feeling at the moment you lose it - are you thinking "ow, that hurts

"? You're not associating it with anything good, that's why.
Try this - punish her by fucking her harder. If you want the pain to stop, you have to drive her to orgasm. If you're getting hurt - that's YOUR fault for not making her orgasm - cause once she orgasms, she'll stop, at least for a little while, right?
The more pain you're in, the more you have to drive her to orgasm. Make yourself believe it's the only way out.
Our mind is the most powerful sexual organ we have, it drives all the others, and we can make it do crazy and amazing things, you can train it. A muscle can get bigger, or leaner, but a brain is more complex, it can form all kinds of crazy patterns.
Here's what you're trying to tap into - there is an element of pain that's tied to sex at a primal level - for example the way the male dog bites the female on the back of the neck so she paralises and arches her back is exactly the same sensation a woman can get when you pull her hair back to make her head drop backwards. Basically there are triggers there to make re-production easier.