Bhagavad Gita, Chapter 2, Verse 25

This Self is said to be unmanifested, unthinkable, and unchangeable. Therefore, knowing this to be so, you ought not to mourn.

The soul can't be seen, can't be grasped, can't be changed. Once you know that, grief has nowhere to stand.

The soul can't be seen, and you can't fully think your way to it either - it's past what the senses or the mind can reach. Which means grieving over its loss was never really on the table to begin with.