Winners never get tired. I highly doubt that we lack energy to do anything; rather, we lack the confidence in whether we are winning or losing.
If you watch fights, of any kind, whether in the ring or actual wars, the side that believes they're winning has the energy to do anything. The boxer who you think is about to get knocked out, suddenly lands a lucky punch and wins. You'll see how energized he becomes the moment he wins. He'll be dancing, jumping around, hugging everyone, giving speeches like no one can stop him, even though a minute ago, he was on the verge of falling.
So, I personally think it's all mental. I live as if I'm always winning or going to win, even though the stats may suggest otherwise. That way, I never lose energy. All I do is win, no matter what.
@HenosT 😈