Welcome to the site and here are the things that you say to her to bring her back to the lightside.
Before and after each battle say the following:
I'll never give up on you, Bastila.
I'm as strong in the lightside as I ever was.
Malak will never let you.
Your dooming yourself to.
Then strike me down.
You are not evil, Bastila.
Now you can see the darkside.
There's no need for me to.
You can reject the darkside.
I was redemmed, Bastila.
You did protect me.
Help us defeat.
You won't, Bastila.
You could use your.
If you enter these answers in the order I gave then you will be able to turn her back and as you have noticed that I only wrote part of the answer but you will see them in the selections. I hope this helps and it is very important that you save your game as much as you can.