I honest to god, from that moment in the TARDIS with the 'friction, fiction, fixing, mixing, Rickston, Brixton' speech right the way through to the 'Songs of Donna Noble' speech, I was in tears, and even now if I only watch that one clip it still gets me every time. How manly, I know!
I know this was posted ages ago, but I thought I'd comment
I'm the same, it's really saddening when she starts losing it
She had the worse fate of any of the companions, it really sucks
Rose got her guy, Martha still had memories of her time with the Doctor but Donna was left with nothing