"I may not always take you where you want to go. But I promise to always take you where you are most needed."

It's about the only promise she can make. Although, what she saw on Trezalore, she supposes she can promise to be with him unto the end.

"The descriptions of this 'Q' I have heard... He sounds a lot like a Time Lord. Or at least what Time Lords would be if they, as a society, ignored their practice of not interfering."