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 Cocker, think ears, eyes and skin before you think training. A Cocker who suddenly snaps when you touch their head, hides, rubs their face on the floor or will not let you near their ears may well be in pain. One who is suddenly tired, weak or off their food may be unwell. For squinting, sudden weakness, pale or yellow gums, or a red, cloudy eye, contact your vet now. Bloat (GDV, a twisted stomach) needs the same speed: a swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling. It can happen in any dog, though it is most common in large, deep-chested breeds. The rest is easy to read as temper, so tell your vet that week, and you will both know sooner.
They'll keep sniffing every doorway. If they start shaking their head, tell your vet.