If she's over 18, get used to the fact that she's an adult, and go find something else to do.
Just give her time. It may take a while, but I bet she will eventually come back. And when she does don't go crazy on her. Treat her like it was okay, because if you get upset with her she will most likely leave again.
I think we need more info on this subject. why did she leave in the first place? and how can you change that? if it was because of YOU, then change needs to happen.
Tell her you just won the lottery
You can't convince her. She has to convince herself. I think if you make her feel as if you accept her and love her unconditionally, she'll realize she should be home with you.
My daughter pulled away from me for 8 years. Thank goodness, we now have a very good relationship, but then again, she has children of her own. That makes a big difference.
She'll come back on her own time guaranteed.