I have this probelm when I was young.
Deep penetration always hits the "BOTTOM" - The Cervix
I suggest you to try her-on-the-top positions: one is my advanced, "Reversed" CAT; other is my 3-point excitation position.
In either way, she will adjust the penetration deep and the penis attack angle so that the glans can exert pressure on her Epicenter rather than her cervix. This is very critic for a deep penetration with a hard penis which can give women either multiple orgasms or intercourse pain. It is upto which spot the hard penis exerts. She must be in a full control situation, adjusting the attack angle to allow your glans to stimulate her Epicenter (the Orgasmic Pacemaker).

But, You have to realize there will be 7 inches of your penis inside her and the rest has to stay out of the vagina. Your glans must get into the "gap" between her Bladder and Cervix. In this way, she may be able to accept upto 8 inches. In fact, my wife always like my hard one to get into the "Gap" - the Epicenter, to give her the Level-7 multiple orgasms. Your penis must be "smart" enough to stimulate the "Gap, " as shown in G-spot.jpg. we discovered this trick along with my invention of the 3_point Excitation Position in 1977. That is why I have advised loving couples to adopt a high attack angle with any love position in the Hard-drive Stage of intercourse when her vagina is fully expanded. Once my glans gets into the gap, I will feel her vagina sucking my glans in every contraction of her Tai-Chi Yang-Kong breathing, and the penis will become even harder.