Accept that sadness is always mourning, for parts of your life that have passed away or that are passing away. Even in a case where your parents or loved ones die, even in a case where your child dies. You are not missing them, for missing creates a longing, a seeking. They are mourning, not the person but that part of their life that has changed, looking ahead to their life without this person.
Sadness is a mourning for your life that has changed. When it changes, all that you knew must change, and much is lost.
When you lose a loved one, you will not see them again in this lifetime, there is nothing you can do about it. And so the sadness can only grow year after year, slowly eating away at your emotions and your ability to live and enjoy what life can bring you. But the moment you recognise it not as a longing, but as a mourning for that which your life has lost, then a tiny part of you can recognise that what you are mourning is change. The life you had is gone, the dreams you had with this person are gone, and change is painful.
Change also always gives us the opportunity to grow and explore new things, to become more than we ever were. If you can recognise you're mourning because your life has changed, it still may take you years, but eventually you will get to a place where that change is something you have embraced, that change can make you love more deeply and feel more passionately, to express the things you feel to the people in your life, it opens you up for potential for the new that otherwise you would not have experienced. It can come with frustration and pain, but it leaves the potential for beauty and wonder, while mourning a loss only leaves the potential for greater emptiness.
This is why it matters that you understand all sadness is mourning the loss of our own dreams, our own ideas, the world we had planned or our attitudes about that world. And that loss is a step in the process of change that can eventually bring you unbelievable, astonishing beauty and some day joy.