The problem with being a (generally) nice person and a (generally) good person is that you can't wish bad things on the people who have hurt you.
And if you should, by chance, become an "other woman" then you are really screwed. Because, in the absence of having met the first woman, you can imagine that she is an evil harpy who does not satisy her man, and so no one can really blame you for, well, anything that you do. However, you know that most likely this is not true. Most likely, she is a nice, normal person who for some reason has fallen into the same trap that you have, namely, falling for this man. Further, you know that he likes her better, because he is not actually with you, he is with her and just likes to "see" you sometimes. But even though you know it is wrong, you like it so damn much that you don't stop. And then eventually, for some reason or other, it ends, and all you can do is hope that either she is an evil harpy and she does deserve it, or that she really doesn't know what he's doing (because he won't change) and all you can think is, "I hope he doesn't hurt her the same way he hurt me."
Same goes if he's actually available...but it's because you are the "last woman," the last person he was with, the one that he screwed over, and you are still looking for the right person, but he finds someone new (which really is not fair; you should get to find someone first). You know he's told her about that psycho he dated, meaning you, even though you're obviously not psycho, just slightly addled because you at one point thought he was a good match for you, and she's sympathetic, thinking that she's found this great guy who's just had horrible luck, and instead of thinking mean things about either one of them, you just think, "I hope he doesn't hurt her the same way he hurt me." (And then you have a minute in which you think, "Maybe she will hurt him the way he hurt me, that would be cool" -- but then you shake your head and stop that train of thought.)
