Okay I managed to completely reset my sources (and Cydia generally). Here's how:
If you don't have iFile, download it from Cydia.
Open iFile.
Navigate to /private/etc/apt/sources.list.d
Delete the file "cydia.list" (click EDIT on the top right, select the file, and click the waste basket).
Navigate to /var/lib/apt/lists
Delete all of the files in this folder except the one called "lock"
Navigate to /var/lib/apt/lists/partial
Delete all of the files in this folder
Navigate to /var/lib/cydia
Delete the file "metadata.plist"
Close iFile and restart Cydia. You should be at the first screen you saw when you first started Cydia: What kind of user are you? Make your selection and Cydia will re-generate the default sources.
Good luck.