That's suck and I'm sorry to read that :(
Maybe you asking kindly these people to credit you might make them cite the source?
Like @Tinfoi said, watermarking your work is a great way to protect what you do. But if it really bother you, maybe these platforms (TikTok/steam/reddit) might have a help section to get unwanted content to be removed?
That being said, it is also the internet. People take things whenever they want, they download, reupload, ect... It's the internet, and we can't do much against it, but hopefully we can limit this, by watermarking and using these help sections :)
TinFoi
I don't think they CAN cite you because there's no Watermark on your Art.
Deimiler
The problem is that watermark makes the art uglier, but it seems like I won't have a choice in doing that.