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 Corgi, watch their back and their back legs. If a Corgi yelps when picked up, will not jump or climb, hunches, or suddenly goes wobbly or weak behind, contact your vet now, because with a slipped disc speed matters. Grumpiness about being touched can be back pain too. In an older Corgi, scuffing the back feet or a swaying walk is not just age. Contact your vet the same day. Bloat (GDV, a twisted stomach) can happen in any dog, though it is most common in large, deep-chested breeds. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now.
They'll keep thinking they're a Great Dane. If they yelp when you lift them, call your vet.