Hi Emm,
I know just how you feel, we had the same problem except the dog is a bullmastiff x rottweiler

all was fine while he was a cute pup but as soon as he was grown up he was banished to the backgarden 24/7 and we couldn't even talk inside our own house or go in the garden. Like yours my dog was a bag of nerves (so was I, my small child was at risk too) thinking this giant maniac was going to get in any second, you could see the fence leaning more and more as he threw himself at it

So I bought a clicker and some dried tripe/ beef and started to clicker train him while his owner was in work (he'd had gone mental if he'd known :rolleyes:) and now the dog is quiet while we are in the garden/house and doesn't react to my dog at all, he only starts now if there are strangers about - which is fair enough. It didn't take long for him to get the idea, the hardest part was waiting for him to calm down enough for me to reward him the first few times, after that he learned really quick ;)
Don't know if this will help in your situation, but it may be worth a try.
Regards
Dill