Sorry you have found yourself here on the website that no one ever wants to find. :( I was in your shoes a little over a month ago.
I am confused by your headline, have you known about this for 5 months? Or did it happen 5 months ago and you just found out?
My WH has done a LOT of digging since DDay and he still has more to do, but his why has to do with low self esteem and feeling like a failure. He projected those feelings onto his family and myself, so he thought we thought of him as a failure. He turned to someone outside because he wanted to escape the pain he was in. It was hard for me not to ask the same questions as you, and wonder what was so special about her that he didn't see in me?
One thing I realized is that he "affair down" meaning there was nothing special about OW except she was the first one to notice him and pay attention. I saw her pictures, she is not attractive, he even told me she wore old ripped clothes, and had nothing really going for her. SHE had self esteem issues too, and together they fed each others egos. Don't even think that OW is better than you. She got the scraps of your WH. SHe isn't better than you, you are the real prize here!! He should be at your feet because of the second chance you are giving him.
That being said, he needs individual counceling NOW. He needs to dig deep and find out why he did this, that is the only way to lessen the chances this will happen again. He should also be completely responsible with helping you heal also. If he isn't up to the task of this long journey, then he needs to have consequences. Stay strong, keep reading and posting, and get him help. MC Me: BW, 28
Him: WH, 28
DDAY 7/4/14 TT till 7/18/14
"Reconciliation means working together to correct the legacy of past injustice." - Nelson Mandela