I’m so sorry to hear this. It does sound like he’s having a middle aged crisis. Im in my mid 40s and I had a brief thing when I was 19 with someone who was 37, he was single but obviously had issues to even go with the teenage me.
In the immediate next few weeks - your focus is on your well-being, that lovely job interview and your kids. Personally, I’d ask him until you have your interview if he can have no contact with you and if he can say he’s gone away with work to the kids. This gives you the headspace you need to focus and also decompress a little from the immediate shock.
once you have the job interview out the way… Id get the legal/ finances in order - speak to a solicitor (you’ll probably find on mumsnet lots of recommendations for a good solicitor near you).
What you want to ensure is the deep dive on your husbands financial situation and assets and to protect yourself too.
id also in addition to the 1-1 counselling support consider if you would feel counselling with your (ex) husband may be something you would feel is useful. No matter what happens next, he will be in your life forever as you have kids. If not soon, but at some point you may want to understand what is behind his infidelity and also how the two of you are going to relate to raise your children.
from a purely financial/ assets/ legal perspective, as others have said - for the time being he is your adversary. The trust is gone. You do need to gather all info as above to protect yourself. This next bit is rather harsh but imagine any text or email or anything even voice notes and voice messages can be used and evidence in any legal proceedings/ divorce. Be careful what you write or communicate from here on in.
what a total shock and my heart goes out to you