Your posture has a big effect on your energy.
Sit with your shoulders slumped, head down for a couple of minutes - how does it make you feel?
Now, sit tall, shoulders back, chest lifted. Now how does that make you feel? I hope much more energised - like the weight has been lifted from your shoulders.
So much of our everyday life is in a forward position - driving, eating, at the computer, checking phones, writing at a desk. All this slumping forward does nothing for our physical bodies, or our energy levels.
We need to balance this out by strengthening all the muscles on the back of our body. Think back of shoulders, whole of the back, glutes, back of thighs, calves.
Strength training and stretching can help hugely. Try to incorporate a bit of both into your daily life.