I'm playing as a female Consular, and was having trouble convincing Bastila to come back to the Light Side. Here's how I eventually succeeded.
The first time Bastila shows weakness, she asks, "How can you still stand against me? Why can't I defeat you?"
The first few times I tried this scene, I chose the obvious option, No. 1, "Now you see the dark side is not stronger than the light." However, after three or four attempts giving "serious responses" and failing to convince her, I made a mistake. I wanted to take a screenshot of my Mac so I could study the dialogue and choices, but when I hit "command-shift-3" to take the screenshot, KotOR reacted as if I had chosen option "3. You can't win, Bastila. Now you must die!"
She responded to the effect of, "I know you're right. Please make my death quick." I then reverted to giving only "serious" responses, and somehow this time I managed to convince her. It was strange, because the first time I played the game, I'm certain I only offered serious responses, yet easily convinced her. In fact, I didn't imagine it was possible to not succeed!
I'm not sure what was different between the first time I played and the second, except that the first time I played a Sentinel, and this time I played a Consular. (I played female both times.) I also played the game more intelligently the second time through, and was much stronger when I faced her. (The first time I played, I actually had to give it my all to beat her; this time I played with kid gloves and had no trouble beating her.) I don't know if it was just a roll of the programming dice, or if my "mistake" actually helped me, but if you are playing a female and are having trouble winning Bastila back to the Light Side, you might want to try making the same mistake I did, and choose the counterintuitive option 3 when that first question comes.