What should we do about our dog?

It sounds like you have a large dog who has been allowed to run the household, (just as she ran her PRIOR household), and so thinks SHE is the leader. With your low level of dog handling skill, this is NOT something I recommend you try to fix yourself. This dog is too strong, and has gotten away with too much for too long. Either find a good private trainer experienced with headstrong dogs, or surrender her to a Mastiff rescue organization. They will have people experienced with the breed, and used to dealing with problem dogs. If you continue the way you have, someone is going to get SERIOUSLY hurt.