There isn't much you can do. Being listed as an authorized user allows you to share the account holder's good credit history (or their bad history), but when you stop being an authorized user, the account stops being reported in your name because you aren't responsible for the debt any more.
Here's an article
https://www.crediful.com/authorized-user-credit-score/
The way to build credit is the same as always; slow and steady, make all your payments on time, keep your credit use low. Maybe try getting one new credit card a year but don't use it much. You want to have open credit that you don't use much and always pay on time.
