Elizabeth Kubler Ross has the best advice around passing away.
There is a process of grief that has been described and recognized..
It starts with Denial, then moves into Anger, bargaining, depression, acceptance ...there may be some comforting information found if you google Elizabeth Kubler Ross...

here is an often quoted poem.

Do not stand at my grave and weep
I am not there; I do not sleep.
I am a thousand winds that blow,
I am the diamond glints on snow,
I am the sun on ripened grain,
I am the gentle autumn rain.
When you awaken in the morning's hush
I am the swift uplifting rush
Of quiet birds in circled flight.
I am the soft stars that shine at night.
Do not stand at my grave and cry,
I am not there; I did not die.