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 teacup Chihuahua, the gap between a bad hour and an emergency is short. Weakness, trembling, a wobbly walk or unusual sleepiness can be low blood sugar: 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. For circling, head pressing or any change after a knock to the head, contact your vet the same day. Do the same for limping after a jump or a fall.
She'll keep climbing the stairs. If she goes wobbly or sleepy between meals, contact your vet now.