Break up, get back together

You don't need cheats or anything. It's the same process as how friendship works. When you for example try to introduce yourself to a pet and they don't like it and you get that little bit of red. What do you do? You make friendly interactions until its back down and now the green is showing. You do the same thing with the red in romance. My girl broke up with her college sweetheart and is now dating the girl she originally wanted who at first turned her down. Now that shes with her things dont feel right. So shes been reconnecting with her college sweetheart. It was full red. Now its down to almost nothing. It doesnt always work but if youre both in a good mood it works, especially if that sim is in a romantic mood.