I researched my attachments. The early ones are in my albums, which was intentional. The later ones were added by accident to messages. I wasted some time looking, but discovered... WE CAN DELETE OUR ATTACHMENTS.
To delete directly:
Go to Settings: Attachments. The list has little checkboxes to the right of each attachment. Check the ones you want to delete, scroll to bottom, click on Delete Selected.
To review thumbnails first:
Scroll down to bottom of Attachment list. Click on Show Thumbnails. Check the little boxes on the right for the ones you want to delete, scroll to bottom, click on Delete Selected.
EDIT: Sorry, this doesn't solve AlexiaLee's original issue. I am leaving this information because it might help others to get rid of attachments, once they are accidentally added to posts.
Apologies, Alexia...