If you got your dog from a rescue group, you should be able to give them a call and they'll work through the problems with you. None of the decent rescue groups ever want a new owner to fail.
In the meantime, I'd stick with bribery. My rescue is extremely dog reactive. I've had him for a year now and even with the custom lessons I still have to carry a small bag of chicken or fish everywhere we go, just in case the wrong dog crosses our path. Most dogs will do all kinds of impressive things for food.
When your dog cowers, stay happy (but calm, some dogs will cower more with too much excitement) and offer a high quality treat. I'd also give something tasty every time your dog goes out willingly just to reinforce the behavior.
Don't get closer if you feel threatened, but don't back down either. Throw small pieces of food at the dog (all normal dogs will eat what's thrown) until you feel less threatened, and then offer the food by hand, letting the dog nibble at it while you move the food (and the dog) toward the door.
Most importantly, pick a technique that works and stick with it. It could take a lot of time, but it will get easier.
Additionally, this is my own personal opinion, but since your dog is showing signs of fear aggression, do not use Cesar Milan's techniques. In my experience this will result in increased fear and a very bad bite.
Good luck! If your rescue group isn't willing to help, pm me and I'll get you some handouts and information from the Calgary Humane Society regarding fear aggression