Pain when initially penetrating and find it hard not to end up bending my penis too much

I (M20) have little sexual experience but have noticed on multiple occasions there is discomfort during sex. I notice this particularly in missionary and when she’s on top and it sometimes feels like my penis is going to snap. It almost feels like I can’t effectively line it up whilst doing missionary leading it to bend painfully . I’m not sure if this is due to lacking experience on my or her part (as the girl I had this worst with was a virgin) or the fact I’m firmly above average in size. Please give advice if anyone has any