my girlfriend doesn't like to have sex because it hurts so we rarely have sex. what do i do?

My girlfriend and I started having sex a couple years ago but every time we do it she says it hurts and usually ends up crying. We were both virgins when we first started so I figured it would hurt at first and eventually feel better but it still doesn't feel good for her. I love to have sex but I don't enjoy it that much when I know she isn't enjoying it. I want her to enjoy sex and want to have sex so we can do it more often and both of us are enjoying it. How do I help her to enjoy sex more and not have it hurt?