Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Doberman, watch their heart and their walk. Tiring early, a cough, fainting or faster breathing while they sleep can be Dilated Cardiomyopathy (DCM, an enlarged, weakened heart), so contact your vet the same day. A wobble at the back or a stiff neck can be Wobbler Syndrome (a squeezed spinal cord in the neck). A Doberman who suddenly snaps when touched may be in pain. Bloat (GDV, a twisted stomach) is most common in large, deep-chested dogs, so skip walks and hard play shortly before and after meals. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now. It is easy to put the rest down to mood. Telling your vet early buys them time.
They'll keep sleeping on your feet. Once a year, take them for their heart scan.