I was another who didn't believe ex would cheat. He also believed he wasn't the type to cheat, and so blamed me for not meeting his "needs". That was the story he attempted to peddle around family and friends - funnily enough its only his immediate family who seem to be taken in.
I refused to ever take the blame. I may have been distracted by my toddler girl but that was me being a good mum to his daughter and was not me giving him my permission to rock our world.
It took me a while to get my head around what had happened and rediscover my strength to get rid once and for all. I had given up my job to be a sahm, and managed to lose my identity along the way. It took a while to pull myself back together again, and I still kick myself for giving up my job and trusting him to always look after my interests.
I now always encourage younger women not to stop working, to find a way to stay in touch with their careers.
I've come a long way since the days of obsessing about what it is that enabled him to take the route he did, scouring mumsnet, chump lady and the like, and I now don't really care - I have a much more peaceful existence without him, and I'm no longer constantly striving to meet his standards of perfection - I am a much more fun and relaxed mum and friend, my house is less stylish (although tidier), I don't always cook from scratch, and I wear clothes that are comfortable rather than striving to be the height of sophistication. Its all about dd and me now.
Funnily enough I can understand the thrill of the chase and the search for excitement. What I can't forgive is the way that he sacrificed our relationship and his long term relationship with out daughter for his own personal desires.
He now bleats on about how much he misses dd, and how he just wants to be the perfect father, and how he is just the victim here.
Sorry - this had turned into a ramble, but my ex was the last person anyone would think would cheat, even himself, and yet he did. It is also entirely possible to discover, survive and grow as a result. If people can take some hope and it gets them through the dark days I'll be glad.