@Helena22 put your best poker face on, give nothing away.
Consult a family law specialist solicitor for advice.
This is a priority, get this done in days not weeks.
If you know of someone through friends who has helped them get a good deal that’s a place to start, otherwise the Law Society has a list of solicitors searchable by specialism & location.
Use the advice to collect information in case you decide to divorce.
The act of gathering evidence of income & assets will be your taking some control in this situation, change the narrative of what has happened to you.
The information gained will help adjust your perspective, as it will allow you - & your solicitor - to scope how you & your children many live after divorce.
Let this adjusted perspective be your friend as you work out what to do, what is best for you.
You have taken a heavy blow.
This has impacted your health & wellbeing.
You are dealing with a grievous shock & demolition of the trust you placed with him in your marriage.
Think not of the OW, she is not your concern & most certainly she is not concerned about you.
Think not of him, he has made his choices.
He chose to jeopardise the trust between you. He is not a callow youth wet behind the ears who did not understand the value of a marriage & family, he is an adult man of many seasons.
The anniversary? When they first shagged.
Get advice from the solicitor now, before Christmas rolls in.
Often an emotional pinch point for relationships where covert matters come to a head. Should anything blow up or blow in, you need to know where you stand so that you may keep a cool head & steady heart.
He has both injured & insulted you, you need space to work out what is right for you, so I suggest that he moves out for a while. No more his carrying on as though nothing has happened. If he quibbles about shifting, put a packed bag by the door & tell him the alternative is that you instigate divorce (& you will have done the prep). The power dynamics between you will have shifted.
Take control, change the narrative, work for what is right for you & the children. Part of your psychological distress is that you have lost control of your life, a helplessness has been forced on you. Taking control sets you on the path to recovery.
This is your life, live it well.