gatheringlilacs Thank you for your comment to me earlier. Yes, things are much better now, and I have learned a lot! 
sternface I recognise the sincerity of your statement, and agree with you that it is a fair question to wonder why a woman would believe a man at all when she already knows he lies.
I can only answer this from my perspective (previous OW, story laid out earlier), but in my case it was rather simply for the following reason:
I believed everybody lied, especially about things that were self-protective, and that the truth was always a hit-or-miss deal, and only possible in 'safe' spaces.
My father led an entirely double life (righteous and upstanding on one hand, physically and sexually abusive at home).
I also lied to him constantly in different ways (mostly to protect myself), eg. telling him I loved him on demand, lying about anything that would de-escalate his violence, lying about my period, lying about feeling ill, and finally lying toward the end about all my plans to leave (and finally running away).
So when I met 'married man', the idea that he was lying to his wife "so she wouldn't get upset", whilst also painting the picture that she was borderline hysterical/violent made perfect sense to me. I grew up doing the same with a violent/hysterical father, but I knew it didn't make me a liar when I said I loved MM.
His 'actions' of love toward me convinced me that the same was true for him.
I genuinely didn't know any of the cliches about cheating men, due to leading an extremely socially isolated life and having no previous dating experience. I also had never experienced any physical warmth or affection in my life ever, and I was so starved for the sensation that I would have done almost anything for it.
I had no concept there was anything better to be had, MM's affection absolutely was the best thing that I had ever felt, it was the pinnacle of joy for me at that point!