This is a huge issue in its own right. She knows exactly what kind of life she wants to live, I think I know (and is inline with hers) but can't feel sure about anything.
People come to realize things in their own time. Personally I think (a lot of) people who seem to figure things out too quickly don't fully realize the magnitude of their decision at the time (not to say that's her case). Figuring it out for yourself is the only way. I was in pain and I came to (a nearer certainty) by writing down my thoughts every single day for months (as others here have also suggested). Everyone has their own style though - writing, talking it out with a psychiatrist or friend(s), praying/meditating...
I almost made this mistake.
Refusing to make this mistake ended the relationship, but it was well worth it.