Hey Jonnyboy, I'm sorry you're feeling scared. I am scared a lot too.
I'm in my thirties now, but I grew up going to church and really believing in God, and there was a time I gave that up because I realized how ridiculous the specifics of it was, and it was right around the time I was 16 too! I remember trying to push the idea of disbelief that would sneak into my head out, because I was afraid if I gave up on Christianity I would go to hell :( It was scary. Now I think a little differently about God in a way that works better for me. Don't be too hard on yourself for not being able to be religious if you can't / don't want to. It's okay.
What is it that's causing you to think so much about this now? Is there some big change happening in your life? Are you worried about someone?
It's a terrible truth we will all lose people we love, everyone we love, if we make it into our oldest age. But over the course of life, love will always come back to us in different and surprising ways. And I think the fact someone as young as you feels so deeply the necessity of love, means you are marked by a sensitivity that could empower you to give back great things to the world.