The OW has the option to stab Morrigan or let her go. Just like a female Hero of Ferelden or any Hero of Ferelden who didn't romance Morrigan. Only a Warden that romances Morrigan has the ability to go through the Eluvian.
didn't you have to keep Morrigan ring on too, what she gives you when romancing her ? i know she says somthing to you if u still have it in withc hunt
Edited
my mistake you don't need the ring
As far I know, only three things are needed to go through the mirror with her:
1: The Warden must have been in a romance with her up until the end of Origins. Breaking up prior to the final battle means you can't go through with her.
2: The ritual must have been completed with the Warden, not Alistair or Loghain.
3: You need to pick the right dialogue choices. Save just before speaking to her at the end of Witch Hunt just in case you mess up. You need to talk to her until you get the option to say "take me with you".
Then you need to say either "Because I want to be with you. No matter what." or "Because I want to meet my son." It is possible to satisfy the first two conditions and still miss the third, if you pick the more aggressive dialogue choices.
If that happens, just reload and try the other dialogue branches until she says she needs to leave and the Warden gets the "take me with you" option.
The ring is unnecessary, though she will comment on it if you have it. As far as I know, all the other options, including whether or not Flemeth was "killed", have no impact on whether or not you can go through the mirror with her