So sorry you are here. In some ways I think finding out long after the fact might be worse or create more problems because you can always wonder about what else went on as so much time intervenes. And whether anything during the past years was real. I would not assume anything she has told you is true, especially as she has been so vehement about hiding all evidence which to me indicates it's worse than she's admitting to. Do you know why she admitted to even this much? Do you think the other spouse has any idea of this? I have to wonder if something is going on over there. If this were me, and I can really only say what I would do, because I'm an extremist anyway, I would tell the other spouse. I think they should know on principle, and why should you be the only miserable person here - let's make the other man's life miserable too. Not only should she know, but it might help you to get more info about what really happened, how long, etc, because between destroying the evidence and the S attempt, I would guess she's lying to you.
I think once again we have the answer to the magic question, can't people have opposite sex friends, and the answer is almost always....NO. Because if you are of a similar age and at all attractive, the inevitable usually happens especially when people are alone. This is why we don't allow ourselves to be tempted. It's foolish to even allow temptation to become an issue. But, again, the answer to this question is almost invariably.....NO.
I am so sorry that you have this mess here to try to pick through. I guess if this were me, I'd ask myself at this point, how do I really feel about this relationship. Do "I" even want to go forward and continue this, even knowing what I know now (and there maybe more as I say). Not thinking about the kids, about finances, about her feelings, about the house, about the extended family or neighbors, or church or anything else....DO I REALLY WANT TO CONTINUE THIS RELATIONSHIP WITH WHAT I HAVE BEEN PRESENTED? My feeling would probably be no. Yours may differ. I usually recommend that people file for divorce at this point because it sets a firm boundary and it lets the cheater know....you are not kidding around, you're serious. Better fess up, come clean, start making amends, whatever you need or want. You're gonna be wondering about this a lot and not only what they were doing and how long, but how this has affected your relationship and family life over the intervening years. Personally, I'd rather walk because I don't like complicated situations - I find they are like a knot you can't untie. That's me though. Be willing to consider anything and be willing to do it FOR YOURSELF. You don't have obligations to her at this point. Not to me, anyway. YOu don't have to be harsh about this but DO consider your own needs and interests FIRST because....no one else did and probably won't. YOu need to do what's best for you, don't sacrifice yourself. Even if you get through this crisis, down the road it doesn't pay off. Do what's best for you once you figure out what that is.
As for her, the suicide attempts are probably manipulative, usually they are. She may very well be this sad and desperate (which again indicates this affair was more than she's saying) and only a mental health professional can really handle that. YOU CAN'T and you should not be expected to. A lot of the time suicide threats or even attempts are ways of manipulating someone and you can't allow this. YOU ARE THE INJURED PARTY HERE AND TO YOU, YOUR WELFARE HAS TO COME FIRST. You only have one life, don't sacrifice it for someone who did not appreciate you. I emphasize this because I think this is pretty common. If she threatens this again, or you think she's going to do something, call the police or EMTS right away. You can't deal with this, this is a professional area. She has to be treated or de-escalated right away by someone who knows how, and she needs to get herself under control. YOU CAN'T LIVE UNDER THESE THREATS. This IS often a way of controlling people, even if they actually are sad and feeling desperate. She has to learn to deal with what she did. Don't allow yourself to be manipulated by her or to feel so sorry for her or fearful that you let it run your life.
This is another reason to contact the other spouse because ....she should know, as I say, and AP should not get a free ride here, and it may help you to get and understand the whole truth. I can't help but wonder what the other spouse thought of this woman "exercising" at their home. I would look very askance at that from the beginning so she may have another side of this story too from her perspective.
Whatever you do be kind to yourself and take good care of yourself. YOU ARE THE INJURED PARTY HERE - YOU AND THE OTHER SPOUSE....NOT your wife and not the AP. Do what you feel you have to do that is the best for you and don't feel guilty about that. Put yourself first here, get enough sleep, drink water (you'd be surprised how easy it is to forget) and try to eat well. Many people either don't eat or binge eat (me). And do come back here and check in - you would be amazed at how many people have been through your situation and maybe have handled it in various ways. But you will get plenty of concern and support here.
As bad as this revelation is, and it's the worst....it may also explains things to you about the past few years that might have been mysteries otherwise. It might provide you with clarity about what you want most out of life and how to get it. Good luck!