Hi Angela
Click on Private Messages
Click on Sent Items on the left hand side of the Private Message Box
On the Blue Bar , on the right of the page you will see the number of sent items and a small box next to the number.
Click on the box. It will select all messages to be deleted.
Scroll down . Click on Selected Messages.
Drop din box will appear.
Click on Delete.
Scroll down and click on Proceed.