There's no reason to think there's an afterlife and it doesn't fit together well with anything else we know about the world. It's hard to even conceptualize it in a coherent way. What is it that's getting reborn exactly? There's no evidence of a soul or stable self that's anything more than a construct of memories. There's no one thing that's you, except this process of a lump of matter evolving into a different kind of lump of matter over time and the mental states that accompany that process.
If no memory is carried over then there's no real connection between the old you and the new you. You're basically two different persons. The amount of subjective misery won't be added up. To every new instantiation of you, everything will be experienced for the first time. They're separate people. So, in my opinion, this is no more horrific than just the existence of other people in this life. Which is pretty horrific too tbf.