I'm not entirely sure, but I have a strong feeling that my dad cheated on my mom with his now fiancée/common law wife. They had been coworkers for years and while they didn't make their relationship public until after he divorced my mom and she divorced her husband, there were strong hints things were going on for longer. As for dealing with it, it was years before it finally clicked in my head, so that kind of helped me. I love my dad and mom and I know that even if he didn't cheat, their marriage probably would have ended around the same time, give or take a few years. For your specific situation you've gotten a shock that happens to everyone: your parents are just people, not the superhuman beings they always seem like. It's okay to be upset with them and to mourn this and possibly the end of their marriage, but remember that this has nothing to do with you.