Hi,
So sorry you have to deal with this. Aggressive dogs are no fun, for sure.
I've trained my own dogs for the past 10 years, including a dog-aggressive St. Bernard. I also worked at a municipal animal shelter for 3 years & as a vet tech for another 2 years. Dog training is a favorite hobby of mine, but I'm not a pro.
My first advice would be to get hands-on help. A dog who's drawing blood is beyond internet advice, IMO. I think you need someone there who can see everything that's going on.
I think Dog Whisperer techniques are fine for Cesar Milan, but they tend to translate poorly to people who aren't... well, Cesar Milan.

In my experience, putting an aggressive dog on its back just escalates the aggression. I've heard of it working once, but mostly I just hear about people getting bitten or the dog becoming more aggressive.
You should check out
www.mysmartpuppy.com. It's a great website run by an experienced trainer, as well as folks from pro trainers, to people who compete in dog sports, to total newbie owners. Everyone is really nice and helpful. They can at least help with some basic advice, and maybe refer you to a good trainer in your area.
Also, check out some books by Patricia McConnell. She's an experienced trainer w/ a PhD in applied animal behavior. She used to do a radio show, & she teaches some classes at the UW Madison vet school. I like "The Other End of the Leash". Jean Donaldson's "Mine!" is also good, and probably the best book for your situation. That one is all about resource guarding.
Kudos to you for trying to solve the problem. I hope this helps.
