Why am I exhausted after workouts that energise other people?

Two people can do the same session and walk away in opposite states. If your body clears the stress chemistry of hard exercise slowly, intense daily training will dig a hole rather than build fitness. The fatigue is a signal, not a weakness.

For slow recoverers, the gains come from spacing hard sessions further apart and protecting sleep around them. Training less often, but recovering fully, beats grinding daily and never adapting.