I can certainly relate. My 11 week old GSD is a land shark big time. He mouths and/or bites everything and can be relentless at times. But it is getting better, slowly and with backtracking on some days but I do see progress.
Have you read through this thread? It has a wealth of information.
Puppy BITING!!! Teaching Bite Inhibition
For myself the following things have been helping the most: exercise, the Leave It! command and obedience training generally, redirecting to something more positive (I carry treats and toys with me at all times when I am home), a schedule including regular naps (he gets super bitey when he is tired or hungry), a boatload of good things to chew on, strong encouragement to chew said chew toys, training for me to learn how to train him better, and patience. Lots and lots of patience.
Hang in there, seek help (professional trainer if needed) and know that you are not alone. GSDs are such a challenge when young but super rewarding when they mature.
Linus van Pelt Haus Juris. My Sweet Babboo, born March 17, 2014.
Madeline Ronan Heart (Maddie) Dec 1997 -- November 22, 2013 Rescue GSD, Sable