Here's something that doesn't get talked about nearly enough: behavior problems are sometimes health problems in disguise. Pain, discomfort, hormonal imbalances, neurological issues, and nutritional deficiencies can all show up as behavioral challenges, aggression, anxiety, hyperactivity, difficulty focusing, or sudden changes in temperament.
Before attributing a behavior problem purely to training, it's always worth a conversation with your veterinarian. A dog who suddenly starts acting out may be telling you something hurts. A dog who can't focus may be dealing with an underlying health issue that hasn't been identified yet. Ruling out a medical cause is always the right first step.