Ok,
Your Mums anxiety issues are not .... your Dads fault
Well they kind of are, coming home and announcing you had an affair but it's all over now, that's ok isn't it? What's for Dinner? Might have a bit of an effect on one's anxiety.
My Dad would have quite happily have just carried on as they were. But in the end they split. He has made it pretty clear that he hates being alone and must have a companion, he wasn't banking on this 'companion' insisting they marry. He went along with it out of fear of being alone. He told us this pretty much verbatim. See various posts above for his habit of not sugarcoating the truth to spare feelings.
I suspect my Dad would take my Mum back in a heartbeat if she'd have him. She is very content on her own. For the sake of a quiet life she has settled into an easy friendship with him. He phones her for chats all the time (when his DW is out - of course) and is planning to visit her next week.
When he got together with his now wife, my Mum was as relieved as the rest of us as it meant he might be less miserable and able to move on and stop bothering my Mum all the time. At my nephew's christing, my Mum fronted it out although it could have been a little awkward that he was there with this new wife and she was still alone. She was adult about it though.
The first exchange she had with Dad's wife was this sneery comment about the christening. My Mum wasn't precious about that in itself, she possibly wasn't that enamoured with the pomp of a catholic christening herself deep down. No, it was more the insensitivity and inappropriateness of the comment, on their first conversation, one which my mum had been steeling herself for, and had hoped would be cordial and polite. She hasn't based her entire view on the woman on this one remark, but it didn't help her to be unbiased when my sister and nan would subsequently complain to her about Dad's wife's behaviour over the years.
My Mum is not an outwardly anxious person, she's very composed. She does however worry extensively about situations which may be awkward or uncomfortable, and rightly or wrongly, the thought of having this lady there and all she represents is enough to cause that worry. The ironic thing is, her utmost concern is that I am as happy as humanly possible, and she is probably frantic with worry that I am worried about her! She has never said anything about her feelings about my Dad's wife at my wedding. But I know she would have an infinitely better time were she not there. When we initially got engaged, I'd flippantly said Oh well, we'll cross that bridge when we come to it' when mention was made of any potential issue. It was my sister who pointed out, quite correctly, that Mum will make herself ill if Dad's wife comes to the wedding. She'll have a string of migraines leading up to the day and her blood pressure will rocket. She won't tell us this though. On the day, she will be as composed and convivial as ever, I certainly do not need to put anyone on MumWatch.
Whether it is her problem or not, my Mum's happiness and well being is more important to me than my Dad's when all's said and done. And when I couple that with the happiness of my Grandma, sister and yes, myself, I conclude that I'd rather not have my Dad's wife there.
Plus, now, if my Dad thought my Mum were suffering in any way, I think he'd do anything to assuage it. I actually believe he would put my Mum's wellbeing over that of his wife.
I'm genuinely grateful for everyone's responses and hope the above goes some way to explaining the situation. I still can't really put into precise words why my mum is so upset at the thought of Dad's wife being there if you can't understand it from the above, which I understand and appreciate many might not.