Thanks everyone for your replies.
I managed to uninstalled all instance of SQL anything, then reinstall SQL Server 2012 and SSMS 2012. Now I can create, modify, and delete databases to my hearts content.
To answer your questions: the original problem was that I was unable to create, modify, view, or delete database. I used to be able to do so with my Windows account on my current machine, then I parked it for over a year, and when I got it back, I was suddenly unable to. Our IT company manages AD so I'm not sure if they made a change that would affect my access privileges to the databases (even though they're all local).
Anyway, thanks for the help.