@amelia12,
If he has cheated on you and his lying about it, the chances are pretty good that he's lied to you before. Has this been an issue in the past?
If lying has never been an issue for you in the past than that is some indication that he may be telling you the truth now. Certainly not a sure thing, but if you're looking for something to support your desire to trust him, that would be one.
We only have your side of the story, of course, but based on it, I would bet that he has cheated on you and is lying.
Interesting that the woman responded "...you need to get your own marriage in order
like I am."
Could mean nothing, but it's interesting. Sounds like she's admitted the affair to her husband and is attempting to fix things with her husband. That she is suggesting he do the same.
You may want to try sitting him down and (without anger-although some tears might help) telling him this thing is eating you up (which it seems is the case), and while you won't be happy about his having an affair you can forgive him if he did and live with it. Tell him it's the possibility that he is lying to you that is eating you up (and this seems to be the case too), and it's damaging your marriage. Tell him that if he has one chance, now, to tell you the truth, that it will really hurt you but you are prepared to forgive him and move on, but if he doesn't and you find out later, you will leave him immediately and try to ruin him in the divorce.
You have to be able to sell this to him, even if you don't mean really mean it.
If he comes clean then you have two choices:
1) You can try to forgive him and move on with the help of a marriage counselor
2) You can leave him.
I wouldn't feel bad about tricking him, if you decide on #2, as he will have admitted to being a lying cheater and you won't owe him anything.
There's no guarantee that (assuming he did cheat on you) this will work and he will come clean, but you may have an opportunity to judge whether or not he is lying.
Most people have a
tell when they lie. Unless your husband is a Grand Master Liar (and his story on this isn't the work of an artist) he will do something when he lies. He'll break eye-contact, he'll blink repeatedly, he'll speak more rapidly or more slowly than normal.
Take a look at this article
http://parade.condenast.com/57236/viannguyen/former-cia-officers-share-6-ways-to-tell-if-someones-lying/
There are plenty of others if you google "physical signs of lying"
Maybe while reading this article or others you'll recognize your husband's lie
tell from past experience, or if you really want to get tricky you can set him up with a question involving what you know or suspect will be a lie (having nothing to do with this woman) and look for his
tell. Then when, a day or two later, you hit him with the ultimatum about the women you'll know what to look for.
Clearly, this isn't 100% fool-proof, but it may go towards validating what now may seem like paranoia or undue suscpician in your mind.
You may never know for certain if he's lying to you but having a pretty good sense that he is or isn't can help inform your decision on how to move forward.
If for whatever reason you decide to give him the benefit of the doubt and stay with him, let it all go. If you can't let it go and it's going to be on your mind all of the time, then my advice is to leave him now.
Good luck