I recently deactivated my facebook account, but want to reactivate it. However, my cousin has been letting me?

sign in under her name so that I can be nosey and see what everyone else is saying. Since I have been going under her account, as soon as I go under facebook ,the computer automatically think I want to go under that account. How can I use the same computer and reactivate my own account without it pulling her account up? I want my own account back. Can someone please help me?