It wasn’t supposed to end like this. They were going to carry on forever, two best mates travelling the universe, seeing amazing things, saving worlds, and just generally being brilliant. Because she’s like him now, knowing and seeing everything, and she understands the Doctor in ways she couldn’t before. His insane-sounding ramblings make perfect sense now.
In a way, that’s the worst part, because Donna’s head is starting to hurt and she understands what’s happening.
Her overloaded brain is killing her.
She doesn’t want to lose her memories, no matter what it costs.
The Doctor doesn’t give her the choice.