I am so sorry he's filed. But, like many of them, they do not tell us when they filed. They do not consider that it is our business to know because they think we will explode, i.e., be angry, and start begging them to try to save the marriage. My xh didn't tell me, but by the time he filed, I was extremely happy to get the divorce over and done with. He tried to take me to the cleaners as well, but he had forgotten one thing...I had worked for lawyers and had access to the law library and had gotten some very sage advice from others on what to do before he filed.
Yes, they do get ugly, selfish and can be done right nasty when it comes to divorce and even though he said at one point he didn't want to do something, he will now. Get your ducks in a row and find a lawyer who isn't afraid to take him and his lawyer on. You want someone who is going to fight for you. BTW, I definitely would state the reason for the divorce is infidelity...why hide it. You certainly didn't drift apart. Do you have something called alienation of affection law in your state? You might want to look up your state and see if it exists. Some states have this law.
Take care and sending positive thoughts your way.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.