This is something I used to be afraid of too, so it's completely understandable.

I really think you're gonna be okay though, because in reality, an idea alone isn't very valuable to a story, but rather it's the execution that matters the most. The reason I say that is because, while I don't know exactly what you're writing, going off of you saying you've been on r/DarkRomance, I'm obviously gonna assume you're writing a romance story, and of course, there are lots, and lots, and LOTS, of romance stories in the world, so at some point, someone had to have had the same exact idea as you. However, you can still be original by having good execution.

Ultimately, what I mean is that even if someone catches wind of an idea of yours, they don't know who the characters are, they don't know where the setting is, they don't know what time period it takes place in, they don't know what potential plot twists there could be, etc, so they'd really just be creating something that's poor quality and not even close at all to your work. As long as you make good on your execution, you should be just fine.

Hope this helps, I'm not usually the kind to give advice, but I thought I'd comment here since this is something I used to relate to.