D.B.
That really stinks. I'm so sorry for you.
I'd cancel the trip - no point in going through with the charade. And tell him why. DO NOT accept that this is somehow your fault for snooping. He has betrayed you by a) cheating and b) deceiving you.
Get some counseling to sort our your feelings and make a plan for going forward. If you have kids, they need to be considered - how to handle them and the changes afoot. You also need to protect yourself legally and financially. Get some good neutral advice so that you CAN think straight. A lot is on your shoulders. So, get pissed, but then get strong and organized. Don't let the anger - however justifiable - get in the way of your decisions.
Good luck. If you've been providing for the family for all this time, you are clearly a strong and resourceful woman. Rely on that.