It’s been said that the difference between human beings and other creatures is that we know that we are going to die. (Yes, I know. Some scientists are challenging that idea, saying there are in fact some animals that have an awareness of death.) My point isn’t to argue with science but to point out that it is part of the human condition to experience profound loss when others die, to have an awareness that our life will at some point end and to wonder what happens next. People who study and practice religions and philosophy and psychology all ponder the mystery of life and death.
As you said, no one knows for sure what happens after death. My grandmother, a minister’s daughter, was sure that God has a mansion with many rooms and that when she died, she would find a room waiting for her with a mint on the pillow of her bed and my grandfather there to greet her. I really don’t know if the story she’d tell us was literal for her. I do think it was at the very least a way to make her faith more familiar and less worrisome.
I think your obsession with the question could be called a crisis of faith. Multiple losses at a young age probably made you question your beliefs more than most people do at your age. It’s normal for a child to wonder where people went when they die and seem to disappear. It’s normal for a child to have trouble coping with the loss. It’s normal for people of any age to be angry with God when someone they love passes. You are still working on finding a way to metabolize your grief.
Since you are a Christian, you may find it more helpful to talk to your pastor than to a psychologist about your concerns. What she or he will probably tell you is that our ideas about what happens next are grounded in faith. Faith isn’t science. It’s a leap into a profound belief that provides solace and comfort.
I encourage you to contact a local pastor. It won’t cost money. It won’t require a phone. Just make an appointment to talk about your concerns with someone who is sympathetic and well versed in the questions you are asking.
I wish you well.