Pure, unadulterated exhaustion.
My life has literally become: up at 5.45am, train, work, train, sleep, repeat.
I'm questioning the need to stick to the plan vs. "listening to my body" and taking some rest. The path of most amateur endurance athletes is to stick to the plan, "more is better", and given I don't think I'm at risk for serious injury, this is the path I'll follow. After all, training through mental fatigue and physical exhaustion now (assuming it's not accompanied by injury) can only better prepare me for race day.
Plus, I had a great swim session this morning, started lethargic but once into the main set, I was working hard as ever. Similar for the evening bike to run session, so I'll keep it going.