Welcome to the bb forum and thank you for sharing your story. I am so very sorry for your loss.
It's clear from your post that you deeply loved your mum and that she loved you too. It couldn't have been easy for you, but I'm sure it was a great comfort to your mum to have you by her side when she left.
I can imagine your great disappointment that cancer robbed your mum of what could have been another five years. Life can be so unfair at times and doctors don't get everything right, you are not alone in this respect. I am so sorry that you didn't get the time.
Whatever you "did wrong over the years" is long gone. Your mum, and best friend, loved you just the way you are. I'm sure she would not want you to torture yourself now. Particularly, as I suspect there is also much that you did "right" over the years.
Perhaps it would help you to try to focus on your mum's life. After my father died I started a journal. I wrote down happy memories, feelings and stories about our life and I added photos. Our life was not always roses but there was a lot of good and a lot of love. That was over 30 years ago and this journal is still one of my treasures.
When he died I missed him so much it hurt and it took time to heal. I still miss him today but it no longer hurts to think of him. My memories bring comfort.
Grieving is a process and it's very individual, so while no one knows when your pain will ease I do know that it will eventually get better. Be patient with yourself. Be kind to yourself.
You hang in there.