hitting her cervix

That happens to my girlfriend a lot. I pretty much agree with what a lot of other people have said. Changing the angle is really the only way to improve it. With missionary, she'll kind of "roll back" (with her legs in the air) when I push in and that apparently feels great to her. With doggystyle I really haven't found a good angle that doesn't cause her some pain if I pound hard.
 
The position of the cervix, its depth within the vaginal canal, and the angle at which it projects into the vaginal canal vary greatly from woman to woman. In some women the cervix is quite shallow and anterior in the vaginal canal and easily avoided. In others, it projects nearly "end-on" into the deepest part of the vagina, and is hard to get around.

I've had sex with women in which it was very difficult to avoid punching their cervix, particularly when it was most prominent during their cycle, regardless of position or angle. In other women, I never contacted their cervix at all, as far as I am aware.

Most women don't particularly care to have their cervix boxed about, and will usually prefer a position and select an angle that avoids cervical contact, whether they are aware of it or not.