This is why what happened to Donna was so tragic. She never believed she was good enough, or important at all, then she learned the truth that she was the most important woman in the universe. And then, she forgot. All she had learned, lost. She learned to value herself and then... Her granddad said "She was better with you." He was right. She lost more than her memories that day, I think she knew that, I wonder if he knew what he was taking from her in order to save her?
I'm still furious about Ten taking Donna's memory without a discussion and without her consent. I liked Ten well enough until that moment. After that, in all the 2009 specials, he was an ass and I couldn't stand him.