Post something, then use your admin user to delete your user. (Click on the user’s name under the post, then ‘purge user’.)
For some reason lemmy requires at least one post for that option to appear.
If that user is your only admin, create a new user, then promote to admin, save your pass! Then use that admin to purge your original user.
Once deleted you will be able to re-create that user. Save your pass!
Would need more info to troubleshoot mail.