Theory:Doctor Who television discontinuity and plot holes/The Star Beast

So the one way to fix the metacrisis is to just let it go, and Donna gets to keep her memories. Why didn't she just do that in Journey's End all those years ago? How exactly does Donna's metacrisis work? You'd think Donna would only have the Doctor's mind up until the point when she originally gained it during the metacrisis (the mind of the Tenth Doctor and preceding incarnations), but she seems to know his future or at least that he was a woman, something that hadn't happened to the Doctor by the time see gained his mind. She also makes references to his face "coming back" meaning she knows he at least had some other faces before this one. Does this mean that Donna gained new incarnations mind's as part of the metacrisis every once and awhile? Or that she gained new parts of his mind once the Doctor reawakened the metacrisis in here? Or something else entirely?