I think Belle does know that the dagger is a fake but she doesn’t want to accept the fact it’s a fake. The mirror only confronted Belle with her own negative thoughts.
Belle’s logic seems to be “Rumple wouldn’t lie to me because he loves me” she can’t seem to grasp that Rumple can lie to her and still love her. Her logic seems to be he loves her and wouldn’t lie or he’s lying and doesn’t love her which is the angle the mirror was going for.
Belle knows Rumple loves her therefore him lying to her doesn’t make sense and she doesn’t want to consider the possibility that he doesn’t love her (when he clearly does love her).
[adrotate group="5"]
All magic comes with a price!
Keeper of Felix