Maybe it's too late for a new reply but I think I'd love to share my way of repairing the romantic relationship.
My Sim broke up with her fiance yesterday because he cheated. I fully repaired it the next day in game. No mod, no cheat needed.
Just call your target to hangout, watching romantic film together to get them flirty. Have some friendly / funny conversations then flirt / embrace / kiss and etc. Keep doing this and everything will be alright. The red bar will be depleted until nothing left and you will just have to build the pink bar again all over again. That's all.
However, I found this thread because I was very perplexed when I repaired the relationship. It was like nothing happened between them at all so I wonder if it's normal because I think it's so weird. Sims have a real short-term memories!? LOL