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 Boxer, watch their heart, their skin and their breathing in the heat. Tiring early can be their heart, so tell your vet. For fainting or faster breathing while they sleep, contact your vet now. A new lump can be a Mast Cell Tumour (a common skin cancer in dogs), however harmless it looks, so show your vet the same week. A swollen, tight belly, retching with nothing coming up, restlessness or drooling is an emergency: contact your vet now. So is loud, fast panting that will not settle in the shade: cool them and contact your vet now. A Boxer who suddenly stops wanting to play, or growls when touched, may be in pain. It is easy to put it down to growing up, so tell your vet early.
They'll still ask for fetch at noon. Say no, and if they flag on the dawn walk too, tell your vet.