Ouch, that sounds uncomfortable! While there is still the cultural message that larger penises are somehow "better" or more desirable, as you're learning firsthand that there's nothing inherently better about them. And that, sometimes, they pose their own challenges.
One thing to try, if you haven't already, is to experiment with different positions to see if there are ones that make vaginal sex more comfortable. However, if you notice that it's just uncomfortable no matter what, it may be better to switch to doing things other than vaginal sex. That way, you two are still enjoying yourselves without putting you in a position that's painful. Do those sound like things you could try?
AS for how porn stars do it, some probably do practice and gets used to inserting larger objects, but porn also involves things like editing, so you never have any idea how much trial and error went into doing so.