Log file won't shrink - suggestions?
Your log_reuse_wait_desc is database_mirroring and you mentioned it currently stuck in a Suspended state

select name, log_reuse_wait_desc from sys.databases


You should find a cause your mirroring was suspended. Try to search messages that include Database mirroring has been suspended in ERRORLOG file on the principal. It could be like one below:


  2019-08-13 11:40:48.98 spid64s     Error: 1453, Severity: 16, State:
  1. 
  
  2019-08-13 11:40:48.98 spid64s     'TCP://xxx.com:5022', the remote mirroring partner for database 'DB', encountered error 5149, status 3,
  severity 25. Database mirroring has been suspended.  Resolve the error
  on the remote server and resume mirroring, or remove mirroring and
  re-establish the mirror server instance.


Using error number you are able to get its description from sys.messages:

select * from sys.messages where message_id = 5149


This case:


  MODIFY FILE encountered operating system error %ls while attempting to
  expand the physical file '%ls'.


That means there is not enough physical space to expand a file on the mirror.

After solving any found errors you have to turn on your mirroring. Run on the principal:

ALTER DATABASE YourDBName SET PARTNER RESUME;


In case you won't be able to find any related errors in ERRORLOG, just try to resume your mirroring using a command above and then look after the database and check your ERRORLOG for errors once the mirrored database goes to the suspended state (if it does).