OP - I've written elsewhere that just about everyone in a long term relationship can understand the temptation of a moment of infidelity- and just about no one on here has not crossed a line in small measure. And there are many many stories here of people who thought they could trust themselves, and allowed themselves to be drawn in. So who can say for sure what lines, when crossed mean what for who?
The single act of infidelity is so often compounded tenfold by the second third and fourth time - by arrangement! And by the lies, the deception, the cunning tricks, the denials when confronted, the gas lighting, the accusations of paranoia and mental torture meted out by the unfaithful partner to conceal the affair.
What you've described doesn't involve those horrible hideous traumas that leave someone destroyed. Instead what you've described reveals that your relationship and your DH is not immune from infidelity in the right circumstances. And in truth, I'm not sure many are - especially over a long marriage. It's just that circumstances don't usually create that perfect storm. I suspect that is where your anger is towards the OW - because she created that circumstance - that perfect storm?
So ...for the woman who got drunk at the bar after her husband left the hotel a day early after a blazing row, who had the attention of a beautiful stranger who said all the right things when she felt vulnerable.....who can say? Only that the amazing stranger usually never appears!
If the same stranger appeared in the the supermarket three weeks later perhaps they exchange a glance at the checkout that let her know she was still attractive - and that she keeps to herself.
What I'm saying OP is that what matters is how you feel about his infidelity- what is forgivable? Now you know he isn't immune and neither was your marriage. I think you either treat it as something to make better or give up on. Either one might be the right answer for you. It's enough to leave but it's not irreparable if you want to repair.