Namashivaya means happiness. When you are worried, sad or distressed, then healing will not take place. We have to shift our mind to a state of happiness, for healing to take place in our lives. Instead, if we worry about our (life) situations, then healing doesn’t happen.
We can say, “Change yourself”, but that change will not happen easily, unless you shift your state of mind. However, when you chant Namashivayam, then your worry state will shift to a happy state of mind.
Please note here that you don’t need to add Om to the chanting (like Om Namashivayam) as OM is inside Namashivayam.