How do I cope?!! I'm so sad...?

Because he's a disloyal, sh!t with no character or real sense of commitment, that's why. You get past this by realizing it takes a real loser to do this to a family he's been with for 6 years, and you deserve someone with brains and integrity. Things he obviously lacks. Leave him to his new woman who won't give him anything (being already married), and know he will be regretting his stupidity soon enough.