If the main TPG email account is the one experiencing this issue, the change or edit of the username is not possible.
You may need to consider setting up the "Advanced Filters" on your "Mail Filter Settings".
With this filter, you need to be specific on what email that will be directed to "Trash", "Spam", etc.
You need to make sure to review the website that's requesting your email address to prevent this.
On the "Mail Filter Settings", you need to scroll down to see the "Advance filters" field.
Are you able to send a screenshot of what you can see on your end for us to analyze it?