MB's suggestions are good ones.
May I ask, do you always use lube? I'd recommend starting off using lube. It could be caused by not enough lubrication on her labia before penetration, causing a friction burn during entry before your cock becomes slick enough to slide against them. That's happened to me about 1000 times, I swear.
I sometimes get labia pains, but they're not usually on initial penetration. She could be failing to relax her PC muscles completely during penetration. That's another possibility for why it could hurt during penetration most of all. Usually when one of my labia hurts, it sort of feels "beaten up" from the amount of pounding that we've done, but it usually doesn't start off hurting like that.
The best positions for me for sex with a penis that's too long, as a woman who is shorter than my partner in both height and vaginal capacity, were the
missionary position and woman on top in an awkwardly named position called
Kneeling Master except I prefer to do that one with my feet on the ground while he's sitting in an armless chair with a back that I can grip, sort of like the
Sybian, if he were sitting upright in a chair instead of lying down on an ottoman. When I'm on top on my knees in the kneeling position, my legs get tired sooner, and my knees will begin to ache after a while. I've found that the leverage with my feet on the ground makes it easier to enjoy myself for longer with less physical exertion and with more control. The advantage of his torso being upright is that I could rub my pubis mont against him as I rode his cock, which made orgasming a lot easier, having something to hold onto with my arms (the back of the chair or sofa) made me less tired and gave me more control, being face to face so close makes it easier to kiss each other's faces and necks or for him to kiss my breasts, and if you enjoy watching yourself then a mirror behind her provides a beautiful view of your cock going in and out between her buttocks that you can watch over her shoulder.
Another position that I loved that is good for preventing too deep penetration and is also extremely hot in the mirror is a variation of
Lap Dance, which is really just the position I described above except with the woman facing the opposite direction. We do this one with my thighs draped over his thighs instead of inside his legs, so that my feet are outside his feet not inside, and we've also done this one kneeling on the bed and facing a mirror, too, the same way, my thighs draped over his thighs, my knees outside his. This has always been a "play with my clit" position for us, because it's nothing without the reach-around, which always makes me throw my head back and rest it on his shoulder when I'm not watching us in the mirror at the same time he is. You can also fondle her breasts with your free hand or kiss her neck or her ear. I recommend that you do!
The
Inverted Jockey is also another good one for limiting depth.
W! Made her buck, ass jiggle and pussy squeeze all at the same time.
