No one is surprised. I don't like the option of divorce, but this broad has to go. You have no obligation to her or her kid. She broke her vows and undermined your relationship by putting herself first.
You can choose to be the doormat for the rest of your life while everyone laughs at you or grow some balls and serve her some papers. It is retardedly obvious. No offense.
Sucks to go through it, but pick yourself up and move on with life. Lesson learned.