Husband having an affair?

My husband came to me 3 weeks ago and said that he wanted an end to our relationship, stunned at his comment because it was so out of the blue and character, When i asked why he had stated that our relationship had been in a bad way for the past 12 months. I would not accept that this was a good reason as there had been no warning signs and my husband and i had always been very affectionate towards each other. I again asked him why ? after a few days of his non talkative ways he says that was having trouble dealing with being a husband and father and needed some time to be on his own. With this revaluation I was only too happy to give him the time that he needed to get his head together as he is Asperges. He moved to the caravan park up the road and i had noticed after a few days he had seemed to be coping ok and still remained the affectionate husband and father. The next night i had a night out with friends in order to unwind after the emotional roller coaster that week and my husbands friends had asked me How I was ? My reply; Im going ok but wish that he was home with us as we all miss him, but at least the time away will be good for him! I then was receiving strange looks and one of the mu husbands friends had bluntly stated that i need to keep an open mind and that a woman was seen flirty with my husband on a previous occasion, I disagreed and said, if he was cheating i would have seen a sign but i had not seen anything really different, only ( we had a spat about the kids 2 weeks prior, a small spat considering the fights we have had). The next day i was on the computer and i notice an icon with a email account and logged in with a common password that we use and i then found a barrage of emails that had been going through my husbands mobile phone, these messages were of intimate talk between them both and even about their love for each other. I felt ill, especially knowing that my husband was going to be turning up any minute to visit. I confronted him about the affair but did not tell him where i found out as he just assumed that one of his mates had told me. He looked devastated that i had found out and remained calm and answered any questions i had ( out of character for him) He stated that the affair had started the year prior for around a month but then he called it off and that she was not the reason he was leaving but that he once again blamed our relationship was bad and had been for quite a while. From the emails i have read she had still tried to instigate communication with my husband repeatidly after he had finished communications with her about year ago, he finally started to respond about a month ago and it seems from the email that she is in love and he stated the same. I do know this woman and her own marriage fell apart around a few years ago.

I suppose I'm asking a question is to why a doating father and husband would do this when he has so much to lose ?

~We have been married for 20 years

~He is 52 and i am 43

~ we have 3 children 20-17-15

~Im an attractive woman who is hard working and I'm pretty out going

~ he has stated he cannot move in with her due to her having 2 sons 15 years and he wouldn't like it. ( he is Asperges so he would find this very difficult as he is not great with children Only his own)

Any advice is greatly appreciated as to why this has happened ! e.g. is this a Male menopause thing ?