I am in a similar situation 21 years of marriage and 4 children. I found out my husband was seeing someone and he left that day as well. It has been extremely hard. I did meet with a lawyer however things are somewhat at a standard right now. Everyday I feel like my emotions are all over the place, I go from angry to very sad. I wish you the best and hope you and your children get through this situation the best way possible.
Definitely consult a lawyer now. I also highly highly recommend "20 Years Gone: A Divorce Story" by Dorian Wright (available on Amazon), as you will find many tips and details to use in your current situation.
I would also recommend looking to see if adultery is recognized in your state for reason for divorce. As unreal as it may seem, some states do not recognize it.