How do I keep faith in God while bad things are happening?

A few years ago my younger sister died in an accident. We were so close, and I struggle so much without her. As I was getting through my grief, one of my best friends died, and within six months, my other best friend was dead too. I also have a heart condition and have been given three months to live. All this has... show more A few years ago my younger sister died in an accident. We were so close, and I struggle so much without her. As I was getting through my grief, one of my best friends died, and within six months, my other best friend was dead too. I also have a heart condition and have been given three months to live. All this has happened so quickly, I'm only 14, so it means I might not even get to see my 15th birthday if the doctors are right.
I realize that God has a plan for me, and that sometimes people need to suffer for good things to happen to others (just like Jesus). How can I keep my faith in God while things like this are happening? How do I shake the doubt that God doesn't exist?
Update: Just to correct some people. I don't think that I will be in heaven with my friends again, because I know that at least one of them didn't believe in God, and hadn't accepted Christ as their savior. That made grieving even harder for me, to think of what they're suffering now.
Update 2: I am in no way scared of death. In a way I'm excited to get to meet God face-to-face. I'm scared for the people who'll be left behind.
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