I'm sorry to say it...but it's likely that your husband is already involved with the OW.

It's a generalization, I know (and apologise in advance to the guys), but men usually don't leave like that....unless they're sexually involved with someone else.
There are other reasons why a man might leave, certainly. But those reasons will be fairly plain and easy to identify. The old "I-love-you-but-I'm-not-in-love-with-you" is straight out of the
Cheater's Handbook.
It's clear that you've had other problems in the relationship. And it's fair to say that you're almost certainly dealing with a Male Midlife Crisis. Affairs are often symptomatic of BOTH those things.
You might begin by researching the "midlife crisis". There's ALOT of info to be had on that just by googling. You might pay particular attention to the physiological aspects of it.
And you need to get the correct information about the affair partner. Your WH (wayward husband) will not tell you the truth. They almost NEVER do.
p.s. One more thing.... Sometimes when a man is having an affair and his intention is
divorce, he goes to great lengths to keep the affair on the 'down-low' until AFTER the divorce is final. The thought process is that he'll be damaged in the legal proceedings if the infidelity is discovered.
Unfortunately, you won't have to read far in the OM/OW forum to confirm that as a common tactic.