Weakness is first a psychological problem not a physical one.
If you are weak in the mind, it will consequently affect your body.
Once you agree in your mind that you are tired, your entire system will work towards the command you have given to it.
You will suddenly become tired. Weak minds produces weak bodies.
Keep your heart with all diligence. Out of it are the issues of life.
Nurture in your mind ideas that empowers you.
Have a great week.