Is there a fix yet? I'm having the same problem with one of my Sims couples - I created them over a year ago, they have been happily married ever since, have five kids, are best friends and right up until Lovestruck, they were the picture-perfect couple. Then their relationship started to decline - at first I thought all I had to do was go into CaS and adjust the attraction system so they would go from "I think you're ok" to "OMG you're awesome". That did work, but last time I checked, that romance bar was maybe one third full, and the wife was having anxiety issues she wanted to talk about.
So I had them do all sorts of romantic interactions, had her talk to the husband to get rid of that anxiety, and bam!, that romance bar was back at 100 %.
Then I accompanied her to work (she's a scientist), and when she got back home, that bloody romance bar was half-empty again because she hadn't seen her husband since breakfast. So more romantic interactions, stargazing, woohoo, you name it.
I don't mind doing all those interactions, and I think Lovestruck added a few really cute ones (I love that they can cuddle and fall asleep cuddling), but it's absolutely insane that the romance bar goes down so fast with two Sims who have always been very much in love and super-happy.
Next time I play, I'll adjust that attraction system even further and click every trait the other Sim has, and I think I'm gonna disable the jealousy system for those two because it doesn't make any sense for them. Also, disabling wants and fears might be worth a try.
