Okay, the facts.
The OW's H knows about her affair and is apparently only staying until she recovers from her undiagnosed illness. If true, that decision buys him some time and gives him back some control over events.
On the other hand, she might want him to stay, never wanted to be discovered and despite what she is telling you, never wanted to leave her marriage after all. But she can't say this to you, because that would blow a hole in this soulmate saga you've been co-creating and this neat little vignette about her marriage being over all bar the shouting paints her as a victim, who will one day be left in the lurch.
I very much doubt her husband is going anywhere.
Friends and your own mother know you've been having an affair.
You told your wife you were leaving because you were unhappy, but you only stayed when the OW wouldn't commit to you.
Most women in your wife's situation know full well that the main explanation for a man's departure, his tales of lost love and unhappiness - is an affair. This website alone is full of threads from bewildered women being spun this yarn and their respondents telling them to wise up and uncover the interloper in their marriages.
What you have to tell your wife won't be a surprise, but it will be a shock.
But tell her you must. There is a strong chance that either she will find out anyway by her own means, or that someone will tell her. The H seems as likely a candidate as anyone.
Before you do so though, think very hard about why you had this affair and what it means. You seem to have got yourself locked into some tragic soulmate epic with its atrocious sequel "The Man Who Fell on his Sword and Did The Right Thing".
It will cut no mustard with your wife if you persist with this tragic love story fantasy, so wise up and face some hard facts. Dig deep and ask yourself what propelled you to have an affair - and I mean really, not the justifications you've no doubt been finding and manufacturing for however many months.
Then see yours and the OW's actions a little more clearly. You were both willing to hurt two families. You've had to tell a lot of lies and have both become accomplished in the art of deception. You might not want to admit it, but you've also no doubt been deceiving eachother to an extent and certainly your own selves.
Then think about your true feelings for your wife. When did they start to change for her, beyond the usual complacency? When did you last feel fully active and present in your marriage? When did you last show her an act of cherishment and adoration? When did you last feel that she was the woman with whom you wanted to spend the rest of your life? In summary, think about what you have been doing to fall out of love with your wife - and when. Don't focus on your wife's actions towards you right now - just concentrate on your own sabotage.
Then answer honestly when this really started. Before the OW and the prospect of an affair? After?
When you confess, take full responsibility for what you've done and be honest with your wife about why you want to stay and for her to forgive you. With any luck, you'll have had a dawning reality that your feelings for your wife sailed when the OW's boat came in - and with her now allegedly shipwrecked, they will return, with superhuman effort and commitment on your part and a willingness to forgive on your wife's.
What no woman with any dignity will accept however is that she is second best - and you would respect her even less if she could. Restore her to being the real prize to be fought for and you just might make this work, but you need to see this affair for what it really was first, stop playing the victim role and start being hard on yourself and your motives.
You won't be the first adulterous couple to convince themselves they are romantic heroes and you certainly won't be the last - and people will continue to be incredibly stupid and cruel for the duration of their ill-fated affairs. Doesn't make either of you evil people, but you do need to get real and face the consequences now - and fight for what you truly want.
I've got a feeling that in actuality, it's not eachother, but your spouses.....