I need a mod that fixes how jealousy works.

Try this one, I think that's what you're looking for:
https://www.patreon.com/posts/ir-cheating-41302780

The only mods I know of that do that are NSFW mods you'd have to ask about on other sites because this aspect is a tangential side feature... that noted, most such mods will end up loading in so many things that you might not want...

It sounds like what you want is a polyamorous trait mod - but one that comes standalone, as by itself there's nothing 'NSFW' about such a trait.

If your brave, you could find the mod that has this trait packaged into a million other things - look at how that trait works (what emotions / buffs it does), and recreate that trait standalone.

There are settings in MCCC that turn off jealous reactions. I ultimately made the decision to use that, because I play with a bunch of different mods that cause spontaneous flirting and other less SFW things to occur, and it just got to be way too much having relationships going on these crazy pendulum swings just from a little wave across the room. It's taken me a long time to find the right balance, but it's getting there. I decided that if I want to play a story where a partner cheats and the other one finds out, I can play out the jealous reaction myself by making them have a fight and break up. So for me it works to just totally turn it off.

Yeah I think that sounds like the best solution to me. Just completely turn jealously off and then have the Sims act on my behalf on how I think they should act. It's a shame it has to come down to this but thank you all for the help here.

You can also try Bienchen's Less Jealous mod if you don't want to turn it off completely:

https://deichschafblog.de/bienchen/sonstige-mods-misc-mods/ (scroll down to LessJealous to download)

Thank you very much, I'll give this a run and report if it works or not.

EDIT: So yeah it still works with the latest patch but my main problem is that they still get negative moodlets and negative Sentiments about cheating, but at the very least the romance bar doesn't get affected.