Installed gnome shell 3 in Natty Narwhal. And now I can't get rid of it, and I can't get back to unity either.

I suggest you reinstall. The gnome3 ppa overwrites the standing gnome files with the new ones. You MAY be able to reinstall the gnome packages manually but this will take time and may give an unstable OS.

But I still can't go back to unity (which is my goal)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

I alsoget this error when i do sudo apt-get upgrade:
Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

oh, and when I select ubuntu as session, it doesn't load. Only gnome shell and user defined session works

The error
"Fouten gevonden tijdens behandelen van: (It means: errors fund during the treatment of)
/var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)"
points towards a problem with the local copy of one of the gnome debian packages. You might try to delete your local file and it will be downloaded again when needed. Just issue the terminal command

sudo rm /var/cache/apt/archives/gnome-accessibility-themes_3.0.0-0ubuntu1~build1_all.deb

and try apt-get update and apt-get upgrade again.

Could try:

sudo apt-get clean; sudo apt-get -f install

Then retry

May help

In https://answers.launchpad.net/ubuntu/+question/154695, you indicated that you wiped the system in which you were experiencing the problem being worked on in this question. If that is the case and you are no longer experiencing this problem, please mark this question as Solved.

you're right Eliah