I have been where you are and this thread has been full of comments/views that also mirror my experience.
I divorced mine after 36 years aged 59.
We did marry as I fell pregnant accidentally a year after we met.
He was seen by everyone as 'a good man/husband/father' much in the same way yours is.
But he never wore a ring. He had one but chose not to wear it.
He cheated the first time after 5 years. It killed something inside me (I know now) but it was dismissed by everyone as 'a bit of fun', my fault as I didn't give him enough attention (yes, that's the old one) even though I was running a business and doing 90% of the toddler parenting. I also accepted his narrative of being 'weak in the face of temptation' and it meant nothing bla bla bla.
So, in the words of Chump Lady (who's book literally saved my sanity when I read it 5 years ago) I ate the s**t sandwich, took all the blame and we carried on building a very good life together. People honestly thought we were a great couple together and maybe we were.
However, I found out about a new affair 30 years after the first and my gut instinct was to just walk away despite the disruption to my adult children, grandchildren (like you I help out a lot) our shared life, the comfortable retirement we were about to start that we had worked hard to get.
Then I started to weigh up what I would lose and what I would gain. Mostly in terms of maintains a close, supportive family unit and financial stability. Also the sunken costs fallacy came into play hugely.
But it changed something inside me, almost like a light going out. At this point I had nothing to gain but everything to lose.
He didn't want to end it (and tried chucking the old narrative at me) but I knew I had to have space from him so asked him to leave. We owned 2 properties near each other so it was possible.
I booked some therapy for myself, went away to a yoga retreat, felt like I was having a breakdown and my self esteem was on the floor.
But the space I took for myself was invaluable at that time and I pushed through the divorce, created my own home, a moderate income and I still do all the family things I used to do just solo now.
My XH liked to have a Wife Appliance plus the status of a long marriage as it signalled to others that he one of the good guys. But he also felt entitled to a bit of 'fun' on the side.
In the past 5 years I've found out about all the other dalliances/ flirting/ short affairs it turned he had had throughout the marriage. Turns out our children knew about some but never told me. A couple of mutual friends had and one said to she thought we had an Open Marriage as he was so flirtatious when we weren't together as a couple.
This post is more to say to you (from my own experience) don't hide from how this makes you feel (horrendous probably) as it almost doesn't matter if he's just indulged in a bit of flirty stuff or gone all the way with a secret sexual basement (Minwalla is good to read about this) it's all about how it's affecting you.
Id get some individual therapy and as another poster suggested, go away for 2/3 weeks and do something that you're interested in. Go no contact for that time.
I won't post too much about how I am 5 years on just that my kids now see it was the best thing I could do for me. I have so much peace within myself now, normal levels of confidence and self esteem and, for me now, I have no regrets. My life is far far better in most ways (except financially).
The biggest issue is he lied by omission and it's only stopped because you found out.
I've quoted this before on other threads but it was the one quote that really cleared my head and it's so true.
A lie is an assumption of power over another. A lie is an assault that attacks not only the dignity of the other person but also their physical and mental well-being.
A lie steals power from the one deceived. It reduces their alternatives.
It causes the betrayed person to act as they never would have acted had they known the truth.
A liar deliberately feeds inaccurate information, and when there are children, the lies reverberate in their lives as well.