IT DOES NOT MATTER IF THERE IS ANOTHER WOMAN OR NOT
THAT IS IRRELEVANT
OP - don't waste time thinking about if there is OW
Don't go snooping for evidence. It doesn't matter because it won't change anything.
Put a steel cage around your heart for now, and get hard-headed.
You can cry all you like later. But you have to take action now, while he if off-guard.
At the moment he is ten steps ahead of you emotionally. You need to get ahead practically.
Don't let him know you are getting ahead.
This man is not your friend. You cannot trust him. You definitely cannot trust him to be "fair" in the divorce. Sadly, many men try to hide money and assets, to cheat their wives and children.
Go and look for the following, and make copies or take photos.
His payslips, and P60's, or if he is self employed his business accounts.
His bank statements including any savings accounts
His pension documents
Evidence of the cost of any big assets, such as purchase price of cars, watches, equipment for sports or hobbies.
Proof of the money you put in for a deposit on the house, and have put in to any expensive improvements to the house, such as extensions.
Proof of how much you have been paying for the mortgage and household costs.
Obvs this will depend on how your finances are set up, e.g. whether the mortgage comes out of a joint current account.
That is all you have to do for now.
Then you can wait, and see what he does next.