Please use this thread going forward: Romantic Relationships decay/become negative fast with no interaction | EA Forums - 4947168
Hello! I had this problem and managed to fix it. No cheats or mods can fix this, it must be done manually.
You need to take your sims on a date. It's because your sims aren't attached anymore and have lost their bond even though it's saying they're still attracted to each other. Try and take your sims on a lengthy date, filled with lots of romantic interactions to build the relationship bar back up. It must be a romantic date, not a get to know each other date. Afterwards, youll have the option to end date cheerfully. Make sure you keep a regular routine of dates, its important! This should fix the issue.
If not, it may be an issue with the fact that you may have cheated their relationship bars when they already drifted apart a little, which has bugged the relationship bar.
