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 Havanese, watch their legs, their eyes and their breathing. A limp or a skipped step can be their kneecap or their joints. Cloudy eyes, or hesitating in the dark, can be their sight. Bad breath, or chewing on one side, can be their teeth. In an older Havanese, a cough or faster breathing while they sleep can be their heart, and pacing at night can be Cognitive Dysfunction Syndrome (dementia in older pets). 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.
You'll keep treating them like the last one. If they panic at the front door, contact your vet.