Oh, if only our two kids got together on this. They would commiserate in existential soup.
For us, DS (now 6.5) had his major crisis last year. He was the opposite of your DD, though. He'd lament that he wished he could BE in heaven where he can live without____fill in blank with pain, hunger, fear, etc. We had to walk a fine line with that one. We told him that while we believe it's true when one is in heaven they no longer have the struggles they do here on earth, we told him God would NOT be happy to see him before He's ready for him to come because DS has a mission to fulfill in his life first. That seemed to help us, anyway. He no longer makes those comments, but now has moved on to talking about his future wife, kids, and careers. He's even drawn a blueprint of his future home!
Remind your DD that God made her
with the ability to be afraid and won't be angry with her for something she can't help. Just as He won't be angry with her that she sometimes thinks mean thoughts, lies, etc. She's still in her corporal state and God
understands that and that's why He gives us good help when we need it. (Such as a Sunday school teacher, minister, best friend, etc.)
Perhaps your DD will feel better if she knew God has a whole, wonderful plan for her life and as she moves from year to year, she is uncovering it and working through her fears. Sort of make it sound like a mystery unfolding...which it is!

“Cast all your anxiety on him because he cares for you” —1 Peter 5:7.