Thank you, yes I have come to the conclusion that actually the OW as a person is irrelevant, and she is grotesque as a human being - the person who told me what had happened also told me that she is a known marriage wrecker, so she has done this before to other women. I just feel so fucking angry that she didn't give a shit about going after somebody who was taken - DH is responsible and at fault, but she is still a heartless bitch. I judge her by my own standards and I would NEVER try to steal another woman's man, never, because I know right from wrong and I care about people's feelings, and any children that could be hurt in the fall out, I can see past my own desires and wouldn't hurt others to get what I want like she did. I know I am a better person than her and that she means nothing to him, sex with her was the equivalent of wanking into a sock I think, she put herself on a plate when we were having huge stresses at home. Our DD had night time separation anxiety from the ages of 8 to 10 and a half, and would cry and scream all night if she couldn't come into our bed with me, which left DH relegated to the spare room and feeling pushed out, meaning huge amounts of stress - I was self harming at one point because it nearly sent me over the edge - and there was no possibility of intimacy between DH and I as we never had any time alone together. She is much better now, went for counselling etc, is sleeping in her own room again and so much better in herself, and DH and I are back in the same bed together as we should be.
As for other reasons why he may have done what he did, I have my own conclusions. Perhaps familiarity breeds contempt, or boredom, and I was no longer new and exciting because I'd been around for years and the OW was different and new. Perhaps because he didn't play the field much before meeting me and was curious. Perhaps he got carried away because his ego was flattered by her attention and he was weak. It may have been a bit of all of those things. Homelife was stressful, sex life wasn't great because of lack of opportunity, resentments about those things, I became invisible to him for a while because I was part of the furtniture at home, he felt sorry for himself, she made it clear she fancied him, flirting started, a work night out and the rest is history. And now here we are picking up the pieces. He has been a classic fool. But I am the one whose life has been wrecked. It would not be an understatement to say that it has left me traumatised. It has affected my MH, I have huge trust issues, I carry a feeling of dread, uncertainty and hurt with me everywhere I go.
And I agree with a PP - I don't think the people who cheat have a clue when they go ahead and do these things how severe the pain is that they cause to their spouse.