Donna doesn’t know what happened to her, there seem to be gaps in her memory, quite large ones in places, like there are things she should remember, important things, but they’re out of reach. So she tells herself it’s her imagination, and she becomes an expert at confabulation when she’s talking to her friends, because she doesn’t want anyone else to know.
She’s always had a good memory, that’s one of the reasons she’s such a great temp. Explain something to her once and she remembers. So why is her memory failing her now?
Maybe she’s better off not knowing.