How to recover data from Safari Sessions extension after clearing history?
If you go to Sessions "Help" (in the gear menu), there are two questions relevant to your issue:


  Why did my saved sessions disappear when I reset Safari or cleared its
  history?
  
  The option to remove website data, available when either resetting
  Safari or clearing its history, will delete extension data without
  warning. Some third-party cache cleaners may do the same. It is
  recommended to take care when using these tools, as any data so
  deleted can only be recovered from a backup.


and more importantly for you:


  How do I recover Sessions data from a Time Machine or other backup?
  
  If you are familiar with the command line, try using the
  sessions-export script. This is the recommended procedure.
  Alternatively, it is possible to physically replace the database file
  with a backup. The .db file under the directory
  ~/Library/Safari/Databases/safari-extension_yoo.david.sessions-deub45nr9t_0/
  can be replaced by a recovered file if the latter is renamed to match
  the file it replaces.


So, if you have a backup, it should be as easy as a copy and maybe a rename.