My ex is a computer "expert". Is he using administrative privileges to remotely control/limit use of my computer?
If you are really worried, backup and then do a wipe and re-install. This will get rid of most unwanted things. Then manually restore important documents and files from backup. If you are not THAT worried, install a firewall/traffic monitor (LuLu, LittleSnitch, RadioSilence) and just monitor the network activities.

The reason you can't "Administrate" can just be that you were not added as an "Administrator" when the account was created. Check in System Preferences > Users &amp; Groups to see if you are an administrator. You can add yourself (if you are not already) as a sudoer and make yourself an administrator though I'm don't feel comfortable explaining this process, use your search engine of choice.

You can list hidden users, check running processes and network traffic in search of something suspicious though it is not always trivial and if you are really that worried, wipe and re-install.