I pretty much agree with what Sam (KS) said. Hard to deal with a rushy horse whose mentally all stressed and focussed on her buddy who's calling to her. Makes for a hollowed out back and that's hard to post to.
The only thing I could think of to do that you could try is to try and slow her posting down by making yourself keep a slow rythm and with the weight of yoru body/seat, you force her to match your tempo , rather than speeding up to be in sync with her. She will be out of sync with you for a bit, but will hopefully slow to sync herself with you. And , hands up. Don't try to force her down by lowering them. I swear, we ALL do this, don't we?