How can I "login" to git?
You don't login to Git.
You do login to a Git repository hosting server, which request an authentication, but Git itself has no authentication nor authorization.
What Git does have is credential caching (check the output of git config credential helper).
On Mac: "Updating credentials from the OSX Keychain": you can check if your old user was stored there, and update it.
If you really want to disable the credential helper, you will be asked your credentials every time you push to a repository hosted on a server requesting authentication.
I had a similar problem with Windows. Updating Credentials Manager helped in my case.
To open Credentials Manager search that setting or navigate to:Control Panel\All Control Panel Items\Credential Manager. In Windows Credentials -> Generic Credentials find your repo and update username/password or delete all that are not needed.