Doctor
I could give you the short version and just say that I'm half you, and that should explain it but fine.
The other half of me, the Doctor that's now half Donna is handling this better because he was already a Time Lord and getting used to human bits apparently aren't as bad as getting used to Time Lord bits. I don't sleep. I can't sleep. It won't shut off. Constantly at a million miles an hour, and all of these new senses. My body is running on empty. It needs sleep, it wants sleep. I can't have any. Everything is burning through my body, and I can't keep up but I'm not dying. My brain should all be burned up by now. But it seems like this place just wants me to be here. Not to mention that I remember things that I shouldn't. Like the fact that if you never met me, you'd be dead. Real and proper dead and not coming back. And then if you're dead than Martha's dead, and then Sarah Jane died, along with her son and that girl Maria. And then everyone in Torchwood died. Everyone in London, people were being taken into labor camps and so I jumped in front of a truck and killed myself for you. So that I could go back and fix time and make it all right again. If I leave people alone then maybe they won't be all dead and I know that usually doesn't stop you, but I know your track record. Better than I ever did before. I'm not going to let people get hurt because of me. Not to mention that everyone is so very busy being all sappy and domestic, that really who gives a flying arse what I'm doing?
Jenny should be with her father. Not me. Maybe one day it'll even all out, and I won't be grumpy and feel like I'm dying and all alone. But it's not going to happen any time soon. I don't want Jenny to get hurt, but she will get hurt being around me, and that's a fact. At least with her father, she's got several of you, and you should get to know her better. Worrying about me isn't going to solve anything. That's the difference between me and you, when I'm lonely, I just want to actually be alone.