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 Jack Russell, watch their legs, their eyes and their ears. A young dog who skips, limps or chews at a hip may have a kneecap or hip problem. A red, cloudy or painful eye can be a lens slipping out of place, and that cannot wait. A dog who has stopped coming when called, or no longer wakes when you come home, may be losing their hearing rather than their manners. 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 their cushion and their watch on the car park. If they stop getting up for the cats, tell your vet.