There is a property " SORT PRIORITY" in every role.
You can maintain 0-100 numbers in those roles based on your roles priority.
If you put '0' in 'Sort Priority' then your custom role will be set as 1st role after logging into portal and so on.
After changing roles priority, remove Navigation Cache in Portal for roles and re-logging into portal and check.
Even you can set decimal numbers like 0.5, 0.7, 1.1, etc...
Regards,