I just ruined my moms life
What did you do to ruin her life?
I told my mom about my father's extramarital affair.
Your father had a secret, but once you found out, it became your secret, too. Your parents have to communicate and decide on how they want to take this relationship forward.
I'm so upset right now. It's really not that big of a deal. They should work this out.
It might not be a big deal to you but an affair can be a big deal to the person whose been cheated on. You should empathize with your mother about why she would feel this way instead of thinking about yourself.