[Little]

Well, if she says you're penetrating too far, just position yourself farther away from her. Her shoulders can be as flat as she wants them, but it doesn't help if her pelvis isn't aligned correctly.
Kneel behind her, then let her guide you in. She can feel better than you can what direction her vaginal walls go in, when you've reached the end of the line, where it feels good, etc.
Of course, you should be open to the possibility that she just doesn't like sex that way. Don't worry, there are plenty of other positions ... though I'm sure it wouldn't hurt to let her know that you've read here that it's absolutely cosmic ;D Haha.
Good luck.