If you are calm and can remain calm then I would seek legal advice asap. Please be aware infidelity can cause PTSD so seek help if you are struggling.
Understand financially what divorce would mean and get your income sorted (increase hours etc) if the solicitor/mortgage broker thinks that is your best option.
Make sure you don’t have sex as std risk. Maybe say you have a gp visit and then you await a gynae referral due to a complication.
Most people are not calm enough to do the above but when he starts lying and gaslighting, begging and pleading, stamping his feet and screaming, wailing on the floor you will not know what has hit you.
Take time to understand where YOU stand financially as that is your priority at the minute. He may have taken advice already, regardless he is not your friend. So keep your plans secret.
If you know it’s over (and I think divorce or reconciliation are both valid choices). Then serve him divorce papers - done deal. Let him be on the back foot. Then you won’t have weeks of wailing ‘I didn’t do it’ as he knows you are serious while you try and get yourself in order. It will also bring him down to earth with a bump. And if you do split it won’t harm your self esteem issues in any way.
If reconciliation is your objective i would post on Surviving infidelity and ask for advice. They will have seen this many times before and will give you advice. The 180 is worth googling. Do not do the pick me dance.
Those saying just ask him. This man may be putting her and (maybe the baby if breastfed) at risk of an std. Asking him may mean he takes it underground. If he is a liar (and sadly many people are liars who prioritise their ‘unmet needs’ over their spouses mental health and sexual health) why would you tip him off?
Also ignore unmet needs theory. Anyone can be tempted by another however it’s your integrity, honesty and commitment that makes you say no. Unmet needs is basically saying his ability to remain faithful is based on his wife’s actions rather than his own beliefs and moral code/integrity. I can’t imagine a healthy individual chooses to break their own boundaries or integrity due to another’s actions. It’s nonsense. It’s an excuse. He has a but in his fidelity.
I remain faithful but not if wife goes to bed early as she has had a baby. Or I’m faithful but not if lady blows smoke up my butt and I’m not getting attention because of baby.
And until a cheater understands why they have a ‘but’ in their fidelity and allow themselves to lie, sneak and cheat they will never be safe as a partner.