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 Brittany, watch for moments that do not look like them. A minute of staring, twitching or seeming lost can be a seizure, and Epilepsy (seizures) is on their record while they are young. Film it if you can. A first seizure, one that lasts more than two minutes, or more than one in a day is an emergency: contact your vet now. So is bloat (GDV, a twisted stomach): 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. A Brittany who stops wanting to run, puts on weight or feels the cold may have an underactive thyroid, which is on their record from prime adulthood.
They'll keep taking your tone personally. If they stop caring what you say, tell your vet.