How can I reenable admin privileges without an administrative user?
You can try whether sudo -s still works (which would allow you to add yourself to the admin group again), but probably it won't.
Do the following
- Reboot into Single User mode by keeping Cmd-S pressed during reboot
/sbin/mount -uw /rm /var/db/.AppleSetupDonereboot
This will trick your Mac into behaving like a new installation (but with your data still intact). Follow the setup without migrating any data etc., near the end you will have the option to create a new user with admin privileges. Make sure to pick a new username here to avoid any conflicts.