Don't leave your wife. You married her for a reason - set aside a day, a week, a month, heck, a YEAR and try to remember WHY you asked her to be your wife.
Remember your children. Put yourself in their shoes. Do you honestly believe that they can grow into happy, healthy people with the knowledge that their father abandoned their mother?
Your stepchild may resent you for 'taking the place' of her true father.
There are so many things to consider here.
Stay away from the other woman. You may think that you'll be happier with her, but is your happiness worth the well-being of your own children?
Would you break your wife's heart over this woman?
And what about your parents? What would they think if they knew that their son abandoned his wife and kids for another woman?
Spend time with your wife, and fall back in love with HER instead. You know it's the right thing to do. Don't be selfish.
Your girlfriend will find some other man who can make her happier than you ever could