I know what film you mean, even though you havent mentioned its name! (Crouching Tiger...)
Anyway, I dont think there is a definitive answer. But the way I looked at it was, she felt so ashamed about what she had done, that she really didn't want to go on living. And she remembered Lo's story so decided to use the waterfall to end it all. However, she also believes in gods and powerful magic, so maybe this is her last and possibly vain hope to wish for things to go back to when she and Lo were in the desert. She doesnt know if it is going to work, so if not, she will just die. Either will do, from her point of view. She cant go on living knowing what she has done. Of course - I could be wrong. It's just my interpretation. We never find out what really happens..