I am so sorry for your loss but so glad for your Grandmother's gain, namely that she gets to be with Christ! I am also so sorry that you are experiencing these dark emotions. It is good that you are seeking to understand. I can assure you that God will use all of this for your good and for His glory.

I read a book a few years ago called A Severe Mercy by CS Lewis. It’s a true story about a young couple who come to faith in Christ after they are married and then not too long after their conversion, the young woman finds out she is going to die. She faces her death with grace and trust in the Lord as her husband struggles to accept it and love her through it and then after her death more struggle and acceptance and peace and even joy on his part. It’s one of those books that stays with you for a long time.

Pslam 90 is a good one to mediate on as well. In it, Moses asks the Lord to help us number our days so that we may present a heart of wisdom to Him.