Hello and welcome to the forums!
This sounds a lot like the probably more common obsession over going to Hell or committing the Christian 'Unforgivable Sin' which is vaguely mentioned in the Epistles. I used to get that one when I was still a child (I've been OCD since childhood). I'm sure the fact I also have Harm OCD (killing myself, pets, and others) and thinking I was possessed by demons didn't play into that at all...
I sympathise- that can be terrifying. All I can do is appeal to logic- what have you done that would be so terrible that you'd have to spend each successive life in such a bad way? I suppose you could sort-of console yourself with the thought that you personally, in this iteration, won't be the one suffering- since people seem to largely go through life unaware of previous lifetimes. I mean, you're you but not YOU, yes?
You're the one that has to convince yourself that the next life won't be just horrible torment. We're all human and we all make mistakes and do shabby things- surely to spend future lives in the torment you fear would require considerably terrible behaviour in the previous life, yes? I'm not uber-familiar with the theology of Reincarnation but it's supposed to be a journey, yes? As the soul gets refined and closer to ultimate knowledge? I'm assuming you aren't a serial killer or anything... so why would you get hammered so badly by Karma?
I'm afraid that's all the reassurance you'll get from me. It's a drug and I'm not a drug dealer. OCD is not a very rational disorder, in my opinion- although when you're in it, it all seems so rational. I know my own OCD fears have seemed perfectly logical- even when I'm engaging in behavior that borders on the psychotic when I'm trying to avoid the feared outcome. Appeals to rational thinking (reassurance) work on the short term but like cotton candy it dissolves without some determination to out-stubborn your OCD. Which is what I find myself having to do- when I can get myself to do it, anyway. Not always as easily done, than said.
Anyway, you seem to think yourself pretty unique in this, but as I started out with, I'll argue that this is just a minor variation to the one people brought up in Christianity get- Unforgivable Sin/Going To Hell. It's identical- just adjusted to reflect your belief in Reincarnation. And as far as I know, it's a pretty common OCD fear- you wouldn't know it in this forum for all the sexual and gender OCD posts, but still I don't think it's a particularly rare fear. And OCD is OCD, no matter how you slice it.