You answered your own dilemma - you decided to not believe in God. And that is true of all atheists - they don't believe in God because they don't WANT to believe in God, and for no other reason, except (as Jesus stated) they love their sins and immorality and they hate Truth.
For you it's different, because you felt betrayed by God because He didn't spare your mom. That's not atheism, that's just anger at God.
But God isn't responsible for what happened to your mom. Jesus said that "satan comes to kill, steal and destroy, but I have come that you might have life, and that more abundantly."
Sin started with Adam and Eve, and it's been a continuous progression, and sin is so extremely wicked, that it hurts even innocent people, and even good and righteous people, so if your mom believed in God, sin in this world caused her premature death through the damaged and failed kidney.
You need to put your trust in God more now than ever, to see you through the pain, because God knows what it's like - His Son died a cruel death on a cross, at the hands of wicked men, and if you think it was tough watching your mom die, how do you think God felt about it?
But Jesus conquered death, hell and the grave, and those who put their trust in Him, as He said, though they may die, yet shall they live again.
One of Jesus' friends, Lazarus, died, and Jesus wasn't there to heal him. One of Lazarus' sisters told Jesus, "If you had been there, he would not have died!" Jesus told her, "I am the resurrection and the life. All those who believe in me shall live again." Jesus asked the two sisters (Mary and Martha), "If you believe, you shall see your brother live again."
I'm telling you the same thing - if you believe in Jesus, you shall see your mother again.
But the choice is yours, but because you've been blaming God (you turned your back on Him, after all), you need to repent, and ask Him to forgive you for that attitude, and return to God, and ask Him to forgive you for ALL the sins you've committed, and then fully commit your heart and life to Jesus.
There is no other way, because only Jesus can heal your heart of the pain of your mother's death, but if you push Him away, He can't do anything to help your heart. Again, I tell you, the choice is yours - and yours alone.
He is ready, willing and able to forgive you, but He won't force Himself on you. He said, "Behold, I stand at the door (of your heart) and knock. Any one who opens up to me, I will come in and live with him, and sup with him, and him with me."
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