I don't think it was a brain-damage moment. As he was speaking, he mentioned that Carla had proposed pretty much out of the blue. Then it clicked that perhaps she had proposed because she felt guilty about Robert. Even though she denied it, he realised that he could never really trust her again as she had lied so effortlessly and constantly to cover the fling.
I don't blame him, she lied and made him sell his business, prepare to leave his family and his familiar surroundings, stole money to aid her blackmailer...hell, I hope tomorrow he goes batsh*t crazy!
...
I've hated this story of Carla giving into Tracey's blackmail from the start but I actually thought that the way they had Nick suddenly realise the extent of Carla's deception was really good.