If you say "I'm too worried..." it means, that there is no need to worry, because she is always tired and there is nothing you can do about it. So you being worried won't change a thing. If you say "I'm so worried..." it means, that you really care about her and you're really worried that she's always tired and would like to help her. Other example: I'm so stressed about my exam - I really don't want to fail it, so I'm stressed. I'm too stressed about my exam - I know everything I need and I don't need to worry.