It's good that you came here and started to open up about what's going on inside of you, and with the relationship. Nothing gets resolved until we face it, talk about it, and do something about it. So let's discuss that.
Regarding "telling", my hard rule is that the truth always matters and always comes first. Sure, he has a right to know, same as anyone else would. But in this case, I'm much less worried about him and more about you. YOU have the personal responsibility to be honest, simply because it is what will allow you to respect yourself at the end of the day. Infidelity, at its core, is about deception, betrayal, and manipulation. All of those things need to stop in order for healing and progress to take place. We can't keep secrets and lies and still heal at the same time. That's like trying to heal with the thorn still in place.
Look, he's cheating, you're cheating, everyone is unhappy and unhealthy, you are abusing yourselves and each other, and at the core of it all is an unwillingness to just "face the ugly" and stop this cycle of pain, abuse and nonsense. That's why, when new people join SI, our first advice is to:
1) Tell your spouse everything
2) Go NC
3) Get some therapy
Married, separated, divorced... it doesn't matter what label you want to put on your relationship right now, the reality is that you have no trust, no respect, no honesty, no empathy, and no self-respect. In other words, the only "relationship" that exists today is abusive and hurtful, to each other and yourselves. It HAS to stop. Unless you're really enjoying the misery and pain.
Alcoholism (addiction really) and infidelity have a LOT in common. They are both responses to trauma. They are both attempts to escape. They are both self-destructive and destructive to others. They both rely on lies, hiding, tricking, using, gas-lighting and lack of self-respect to even exist. They both end badly in all cases. And they are both 100% treatable. You can recover from this trauma, but in order to do so, you must stop participating in it. An alcoholic can go to AA 7 days a week, but if they keep drinking, then what's the point? Same here. This shit-show has to end. You can't control your husband, but you can control yourself. So end it. Go heal. And if you still want your AP or your husband at the end of that, then at least you will be making that decision with a healthy outlook and with your self-respect and integrity intact.
My advice? Put the relationship (the marriage, but the AP too) "on hold" for now. Neither of you are in a good state of mind to make those kinds of decisions (R, D, S) at the moment. In the same way that you need to put on your own "air mask" before helping another to do the same, you both need to heal individually right now before you are even capable of healing as a couple.
I know it will absolutely devastate our family.
Are you under the impression that your family isn't already devastated, or that remaining in the messed up marriage you have now won't do the same thing? You don't worry about the paint job AFTER the car crash, right? The best way to make sure your family is completely demolished is to keep going the way you are now. The damage is already done, the goal now is to prevent further destruction.
As a WS myself, I can tell you that, as scary as it is to think about just unboxing all those secrets, lies, and airing out "who we are and what we've done", it is SO MUCH BETTER to get it all out. For me, it was as if 1000 pounds of weight was lifted, and suddenly, all the fear, all the uncertainty, all the shame and blame and hurt... was all gone. Putting it all out there stopped the bullshit and pain in its tracks, and in its place, was a sense of calm, of surety, and the ability to stop worrying about what MIGHT happen and instead control what DOES happen.
This is not an easy thing to do or face. But the hardest part of it all is getting over the fear of what may happen. Once told, that fear is replaced with reality, and reality feels damn good after all the deception and betrayal.