I think the key here is where you said: "she can focus when she wants to and if it's something that's she is interested in". Our society so often forces kids into situations (especially public schooling) where they are expected to focus on things that they are not interested in, and then diagnoses them with "disorders" such as ADD/ADHD when they respond in a perfectly natural way (by becoming restless, defiant, etc).
She is an 8-year old. She's simply not going to sit there for 3 hours studying math (unless she's super into math). There is nothing wrong with her for responding this way, and you don't need to worry about it (although you should be concerned about how you are going to create a situation that is more fulfilling/stimulating for her) . You used the phrase "make her care". This is never going to happen. If you try to make her care about something she doesn't like, she's only to get more restless and defiant.
The best way for her to learn and grow is to support her in those activities that she is interested in. What are these areas that she is interested in that you say you can see her focusing on? How could you work on stimulating more interest in these areas, and using them as a platform for learning/growth?