Get administrative rights on your own SQLExpress instance

When installing Microsoft SQL Server Express edition, only the user that is installing the database server software is added to the sysadmin role.  Other administrators of the host or domain don't have sysadmin rights on the server.

Being an (other) administrator, you can obtain the sysadmin role by running a cmd-script as indicated on this post.  After successful execution of this script, the current administrator user is added to the sysadmin role.  So you can log in using e.g. the SQL Server Management Database to create a new database.