Hey MLebowski,
Thanks for using Apple Support Communities! I understand that you're being prompted to sign in with your girlfriends Apple ID, and would like to be able to have your Apple ID set up on your iPad. I am happy to help.
To start, purchases made, or apps downloaded using your girlfriends Apple ID, are linked to that Apple ID and not the device. You would need to re-download any apps you still want to use, with your Apple ID.
To remove your girlfriends Apple ID, restore the iPad as new. Then, when prompted sign in with your Apple ID. You can find the steps for restoring here: Restore your iPhone, iPad, or iPod to factory settings
Thanks again, and have a great day!