If you are the other woman, should you tell the girlfriend/wife?

I’ve recently ended a painful relationship and find myself obsessing over whether I should tell the other woman. I’ve known this man for 20 years (he’s my hairdresser – another dilemma for me now!), and he pursued me for 2 years when we both split up with our partners. I eventually gave in a year ago and went for a drink with him, even though he told me he had recently started seeing someone. He said it wasn’t serious blah blah and it was “just a drink” (I know). I thought it would be an awkward one night stand and we’d go back to cutting hair, but we fell madly and crazily in love. He was with me all the time, stayed at my house, met my kids etc. I presumed he had ended it with the other woman, but later found out he hadn’t and this went on until I ended it and told him he had to decide who he wanted to be with.

It was my therapist who actually raised whether I felt some obligation to tell the other woman. She knows nothing about me, especially as she’s likely to start introducing him to her kids. If I’m honest, part of me wants to hurt him, but the other part does feel guilty for her that she has no idea, and she has fought for someone who has lied and cheated on her for a year, and will certainly do the same again. I’m not worried about the potential retaliation – he’s too pathetic to come after me – and feel this might be the final act of closure. But there’s also a part of me that thinks, should I just leave well alone and think this isn’t my business and she’ll eventually find out?

Interested in your views/experience on what I know is a very complex and contentious subject!