How do you stop feeling like something is your fault?

So my grandmother died in August and we were best-friends. I told her everything and we were as close as two people can be. She was sick for a long time but nobody new for a long time, then she started to get sicker and we found out, I tried to get her to go to the hospital but she refused. She layed up in her room for about 2 weeks alone and dying, I didn't know she was going to die but I also knew she was sick and I just left her up there alone to die by her self, I feel like I let her down and I let her die. I cry myself to sleep every night and think about this all day and night long. I feel like a horrible person and like I let my best-friend die. I've been through so much lately and it just keeps building up, more and more stress builds.
I hurt myself and feel like a killer because of this.
I need help.
Is it my fault? did I let her down?

It is not your fault and you didn't let her down. God chose for it to be her time to go is all. And she may have had a feeling that it was her time and that was why she didn't want to go to the hospital and she wanted to stay in her home. I understand your grief and I am so sorry for your loss, but you should pray to God about the guilt your feeling and that he help you let it go. Your grandmother is in a better place and I'm sure she wouldn't want you feeling guilt over things you couldn't help. I'll be praying for You

I saw her getting wheeled down the stairs, half dead </3

THE WORLD COULD CAPTURE YOUR THOUGHTS, AND THE THINGS SLIP BY THAT ARE IMPORTANT. PRAYER AND IF ANY REPENTANCE WILL BE FORGIVEN BY THE FATHER GOD. PRAY AND HAVE PEACE IN JESUS NAME.

Oh, my heart really felt for you smile for the camera when reading your post. Firstly it is not your fault you are only 16, how can a sickness leading to death be your fault. It is a triple whammy for you losing your Nana, your best friend then being overwhelmed with guilt. Sadly guilt is a part of the grieving progress but your guilt is unfounded and I'm sure it would grieve your Nana's heart if she knew you were blaming yourself. She was a mature woman who may have been very tired of struggling with illness so she may have chosen to just let nature take it course and go and be with her Saviour, it is prue joy to be with God and Jesus yet a deep lost for us that are left behind here.
You did not let your best friend die, please stop thinking such thoughts I feel that you are being deceived in thinking that, when we are most weak and hurting the devil is prowling to attack us and undermine our faith as believers.

Grief is a long journey please be gentle and kind to yourself during this time, your heart is mending from such a deep lost.

My prayers are with you may the Lord hold you in the palm of His hand while he nurtures, comforts and loves you through your grief and loss.

You have no reason for guilt, we grandmas know we are always loved.
Your grandma's desire for you would be your every happyness, never guilt.
You need to mourn your grandma, but do not regret, as regret steals joy from you.
Praying for you in Jesus, that you will be filled with His joy and love, the same joy and love your grandma would want for you, and the peace, comfort and care you need as you mourn her.

Huge hugs and God bless
pickles