Because Carol (Helen Hunt) is so upset during this scene, it's difficult to understand what she's saying. Also, I think because she's angry and upset, SHE may not even know what she's saying. Here's my interpretation (as best as I can understand):
"Leave me be" - she's telling her mom to stop bothering her, and leave her alone. To mind her own business.
"Why are you doing this? What is it you want?" - she is asking her mother why she keeps trying to help her. Unfortunately, even the mother isn't sure what she's doing, or how she can help. Carol is trying to tell her mother, I know you want to help me, but you keep asking me the wrong questions, and telling me to do what I don't want to do.
"I hope that getting me thinking about everything that's wrong - when all I want is not do that - has some purpose" - This is a complicated sentence because there is a clause in the middle.
The main sentence is: "I hope that getting me thinking about everything that's wrong . . . has some purpose" (she's saying, Mom, you are making me think about everything that's wrong. I don't know why. I hope it has some purpose).
The clause in the middle ("when all I want is to not do that") means, Mom, you keep making me think about everything that's wrong, and I DON'T WANT TO DO THAT! I don't want to think about everything that's wrong!
I hope this helps. It's a very touching scene, and I love that in the next scene, when Carol and her mom are walking down the street, and Carol kisses her.