You are going to have to take it.......you messed up and your biggest mistake was not telling her as soon as you found out this woman was having your child. I love my husband but I could absolutely not forgive him cheating, AND finding out he got someone pregnant, AND they'd had the child AND he'd kept it a secret for four years!!!!!!
Don't worry about who she's sleeping with, you got to sleep with someone else so all's fair. Take care of your kid, move on, and hopefully learn from your mistakes.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi