Saddest is people you live being in pain or suffering or dying whilst you helplessly and uselessly stand by
My sister in law is in the terminal stage of cancer, it's overwhelmingly awful and I seem to be already grieving for her but also my brother, my nieces and her parents.
My mum died a year ago, and this sadness is also still within me
The way I cope with sadness is to take a break when it threatens to overwhelm me a bit, allowing myself to feel all the feelings and cry, and then to deep-breath and continue as normal.
I guess it's just life. The more people you love, the more you have to lose, the more you can hurt. But isn't it wonderful to love people, and isn't it better to have loved and lost and to never have loved... all those cliches.... but it helps.
To me, mourning is the flip side of caring about someone. And it's a fact if life, we will all lose people we love.
Just be kind to yourself OP and count your blessings, the good things that were and are