furniture/place guarding

Question
My 21 month old female rottweiler mix continues to have serious guarding issues.  It took many months to help her over her need to guard her food bowl. Now I need to address her guarding of the sofa, which she sleeps on.  She tolerates my sitting on it fairly well, but becomes agitated and protective  - running over to me, growling, and menacing - when I go near it to straighten the throw or fluff a pillow. She will bite if she thinks it necessary. Even if I crate her while I do this, she barks and shows many signs of distress while I am pottering with the sofa.

Answer
Your DOG is running your household.  Given her food guarding proclivity in the past, my best guess is that she is trophying objects as a result of inadequate supervision/nutrition as a neonate, which is now generalizing to objects in your home.  Some of this must be essentially fear (anxiety) driven and is an artifact (most likely) of her less than adequate beginnings (before you met her), coupled with your (clear) inability to provide emotional security (not your fault.)  This dog is a clear and present danger to you; she requires the intervention (because of her active dog to human aggression) of a certified applied animal behaviorist.  There is NO WAY I can safely (or ethically) help you to obtain psychological/physical control of this dog, because I am not able to SEE this behavior in person, evaluate her overall temperament or willingness to "please" (using positive reinforcement), nor can I question you at length regarding her time with you or obtain veterinary records.  Call the veterinary college in your geographical area and obtain referral to AN EXPERT; the dog does NOT need to be seen in your home, it can be done in a clinical setting.  Even if you have to travel several hours in each direction, it's well worth it.  The dog requires calm, consistent leadership (requiring you being instructed in positive reinforcement training) and behavior modification protocols (to reduce her anxiety and self perceived need to get control in your household.)  Without this, SHE WILL most likely bite you.  DO NOT hire a dog trainer.  While many trainers are caring and educated and sophisticated, MOST are NOT.  It will worsen the problem.  You need an expert.  FIND one ASAP.