By the way, which diagonal you are taught to post on - would depend where you lived and who you took lessons from.
In Portugal, Spain, Russia, other countries, you would be told to post on the inside diagonal.
What you have to remember, is that until you are a much more advanced rider, all those debates about diagonals and WHY are not important. Just do what your riding instructor tells you to do.
In general, if you are taking riding lessons in the US, Canada, Mexico, the Islands, Britain, you will be taught to post on the outside diagonal. You may have misunderstood the other instructor, but even if she DID teach you to post on the inside diagonal, it's very, very easy for you at this point to catch the other one.
Now...AT FIRST...the first couple rides, no one much cares WHICH diagonal you catch, especially if you're having trouble catching that rhythm. They just want you to 'get' that rhythm, so you're going UP-DOWN in the right rhythm.