Get screen shots of the messages, email them to yourself.
Get all the financial info you can.
What he earns, wage slips, joint and single account information.
Pension info, asset info, mortgage info if you have one.
Then contact a few family solicitors in your area and see if any do a free half hour.
Then go along and find out where you would stand if you were to separate.
I'd keep it close to my chest for now while I gather all the evidence and things I need.
You need to look after yourself.
Keep yourself hydrated and your sugar levels up.
Sugary tea, soups, ice lollies, anything to keep you going.
You are in shock and will have a crash soon enough so you'll need this to ensure you can carry on.
Do NOT go stalking her on the internet.
Do NOT do the 'pick me' dance with your shit of a husband.
Take your time, bide your time until you are ready to make a decision.
No knee jerk reactions.
Do try to talk to a friend, a non-judgmental one who won't go blabbing or judging you if you stay. But will give you hugs and lots of wine to drink as and when you need them.
So sorry this is happening.
Doubly awful at this time of year!
