To the OP, your dog isn't 'worthless...' He's doing exactly what he is supposed to. He's a good companion for your girlfriend, and that's probably 80% of the reason she got him. A dog acting as a 'guard' dog is a benefit, make no mistake, but heck for $50 you can buy motion detector alarms and install them around your home at the entry points and cover the same needs.
These are my two guarddogs. I rescued the cattledog from the shelter when he was young and the black lab was a stray pup.
The cattledog was simple to train and is very instinctive and a great guard dog; he is very calm and obedient and just knows who belongs and who doesn't and he's a FIERCE fighter - a few dogs have picked on him at the dog park and regretted it. And he has an unbelievably fierce bark. He shares the [excrement] out of strangers when he barks at them when they come to the door! And he also barks at deer, squirrels, and other wildlife as if to say "If I were ONLY outside, you'd be mine!"
The younger lab coincidently looks just like the older cattle dog, but she has a wildly different personality. She is excitable, playful, digs, jumps, and just loves everybody.
Part of this is my fault because I got the cattledog when I had time and space to train him, but got the lab when I was very busy and didn't have the time or space to properly exercise or train her.
When I come home from deployment I will spend more time with the dogs and training.