Mac Mail Issue

I read my yahoo email w Mac Mail. The problem is Mac Mail has created separate Sent folder --- and one of those sent folders does not show up if I go to yahoo and view my mail in my browser. In Mac Mail I have Inbox, Draft, and Sent folders. This sent folder has only a few files. Down below with my saved folders (under the Yahoo heading), there are two add'l Sent folders full of sent emails. How do I compile these into one folder -- capturing past sent emails and future sent emails? Thanks.