It usually means that your dog thinks she is alpha over you. That's why she's protecting you. You need to let her know you are dominant. Don't let her go through doors before you, eat before you, etc. and the other things you are trying. There is a good book about this called "The Dog Listener". Very informative.
Jan Fennell the Dog Listener & Amichien Bonding - Dog Training Maybe you can get some more ideas there. Hope this helps! You can also get her books on Amazon. That's where I got mine.