Why is my German Shepherd still so shocked and stunned?

Sorry to say this but you've gone the wrong way about introducing the cat to the dog!!! He probably wants to attack it but because he's trained and well behaved he daren't do anything and therefore the shaking and the ignoring of the cat is his way of saying - get it out of my sight before i rip its head off!!! If, in three days time, the dog is still the same then you'll have to take out the cat for a few days then re-introduce it differently - don't just plonk it down and hope for the best - you're very lucky your GS is a bit timid!!! Take the cat, in your arms, and sit in the same room as the dog, holding the cat all the time. Let the dog sniff and nudge the cat whilst its still on your lap but, whatever you do, DON'T make a fuss of the cat, your dog will get jealous and all hell will break loose! Try holding some bits of chicken in your hands when you bring the cat in and do a one by one sharing - bit for dog bit for cat, bit for dog bit for cat etc. Its boring but will save a lot of trouble later on trust me! You cannot force them to like each other and the best you can hope for is ignorance on both parts - my three dogs are s**t scared of my cat ha ha but then he is a big brute and totally unfazed by dogs and their teeth!!! Strange fella! They all get along now, they didn't when they were young so perhaps you should persevere - i did all those years ago and i still haven't got harmony ha ha but at least they're not tearing lumps out of each other! The cat rules this roost and he'll whip the claws out if he thinks the dogs are taking liberties with him at dinner time - not such a bad thing i suppose as i know my dogs won't chase cats in the street or park - they now think all cats are like the one indoors so are extra wary ha ha. All you can do is have lots of patience and nerve and see how it goes but if your dog stops eating and gets very depressed the cat will have to move out! Theres nothing worse than a nervous, jumpy GSD!!!