Is he staying elsewhere or has he moved back in?
Claiming that it's fixable and suggesting counselling is faithless twunt-speak for 'the ow's got cold feet and I'll be out in the cold on a limb if I don't suck up to sarsiem and get my feet back under the table'.
And then he had a dim wattage lightbulb moment - 'I know what to do - I'll be all sweetness and light, make her a nice tea, and suggest we can easily get the train which I chose to derail back on track... Sorted'.
If he suggests counselling again, flash him your most sardonic saccharine smile and say 'Marriage counselling for the three of us? But it wouldn't be right to exclude OW's dh - will you ask him when he's available to attend sessions or shall I?'.
You may be feeling pathetic at the moment, honey, but it won't be long before outrage sets in - how DARE the pair of them put you and her dh on trial, so to speak, while judging whether or not to bin you and how FECKING DARE he think you're naive enough to believe that they came to this mutual agreement without having first tried each other out in the dressing room bedroom department.
Is this how he shows his appreciation of his loving, loyal, and devoted wife of the past 20 years?
My blood's boiling at the nerve of him - and her! 
I'm not saying that counselling should be ruled out, but you've had a helluva shock and you need time to process the enormity of having your world turned upside down because he couldn't keep his flies zipped
At the moment you're on a rollercoaster ride of emotions which will temporarily cause you to not know whether you're coming or going, but take heart, honey; the brain mush will firm up and you will be able to make the clear-headed decisions necessary to reassess and redefine your needs and work out whether you want your future to include or exclude him.
In the meantime, use this thread to vent and offload and regard it as your personal but entirely anonymous diary which charts your progress - in the not too distant future you'll look back at it and wonder how you survived came so far so fast.