This is a tough situation.  And, basically, when a man is in this mode, he's in "survival mode."  That means, that as long as he thinks he can make you feel like the bad guy about your suspicions, he'll continue his bad behavior.  Now, I've read over the advice here and you certainly do have options.  I'd like to suggest another way of looking at this that might have a healthier outcome...perhaps not the one you want right now, but maybe healthier situation all around.

First, you should calm yourself.  If that takes sitting at home one night and balling your eyes out, then do it.  Get the raw emotion out of you, but not with him there.  If it takes being with a trusted girlfriend, then do that.  But, not publicly and not where he can hear or see you.  This is a good, healthy release.  It's kind of a cleansing so that it's not held up inside of you waiting to burst.

Second, you approach him on the next night or day when you are both home.  In a calm, unassuming manner, sit at the table and ask him to sit with you.  Let him know you want to talk.  Don't make it an option or up for discussion.  NOTE:  When a woman is calm and cool, this strikes fear into a dishonest man and he'll most likely listen.  Hysterics only play to their favor.

Now, he won't expect this to be the first sentence out of your mouth, but you've got to say it and without raising your voice or shedding one tear.  You simply say to him that you know he has been seeing this woman for some time.  Tell him not to argue, it's pointless and it would be easier for both of you if he would just come clean about it.  There will be one of two things that happen here: 

1. He will begin to raise his voice and hurl insults at you - at that point, you simply get up to walk away and say to him, directly as you are leaving, "Let me know when you're ready to have a serious conversation about this, then."  At this point, you calmly grab your purse and keys and walk out the door.  No bags, nothing (make sure you are dressed with makeup, etc., no sweats).  Go for a drive, go shopping, go hang out with a friend, watch a movie, whatever.  DO NOT answer your phone...no matter how many times he calls.  Don't even listen to his messages until several hours later (this is so you don't second guess yourself).  All of this is being done so he understands the seriousness of how you feel - it's important because your loving ways have been unsuccessful thus far.  Do not return home until you have been gone for at least 4-5 hours or longer.  Do not stay out all night, however, because this only gives ammo to his cause (so, in other words, if you don't leave the house until 7:00 at night, don't be gone past 11:00 or 12:00).  When you return home, if he is calm and ready to talk, then do so, but sit apart from him and stay calm.  Let him lead the conversation and do not give away your hand.  If he is not there or unwilling to speak, simply go to bed (this is hard, but you've got to do this for your own sake).  The next day, or day after he'll either be ready to talk about it or you'll know his intentions by lack of compassion - either case, you know what hand he's holding.
2. The alternate reaction he may have is one of shock and disbelief.  With your confident conversation toward him, he may well assume you KNOW for certain and there's no use in hiding it.  He'll either admit it or remain silent not knowing what to say.  If this is his reaction, then let it be silent for a while.  Be prepared for what he may say next because it WILL sting regardless of how long you've suspected it.  From there, you can talk through what to do next and how you work to get past it.  My hope is that he'll realize it's been a mistake. 

Men often under-estimate their spouse because they become accustom to the unconditional love a wife can often give (not to say men don't do this, but we're not talking about that right now).  I've gone through a very similar situation myself, so if I sound certain of all this, it's because we had a long talk about things after we decided to fix things and he was quite honest about his perspective.  Something for which I became thankful, later.  If we don't show them that we are strong with or withOUT them, then they take it for granted.  That something inside them starts to think that we'll put up with anything because our love is so great for them.  Well, when you show them that is NOT the case, it freaks them out.  All of a sudden, they're faced with knowing they DON'T know it all.  That's where you have an advantage.  However, staying CALM is key.  Your confidence MUST out-weight your emotions.  This is also about you reclaiming your own dignity.

As for the other woman.  There are two main reasons I can give you for NOT contacting her at work or any other time.

1. If she knows about you, then he's either lied about the relationship you have together and nothing you say will convince her otherwise.  In fact, it may give more credence to what he has said to her.  It won't do anything other than frustrate you further and make you feel even more helpless.  It could even lead you to do something that is humiliating and that doesn't accomplish anything other than regret.
2. If she doesn't know about you, it's nothing to do with her.  Talk with your husband first as I mentioned above.  Find out what's going on there for sure.  Then, you can call her or perhaps, leave a note on her car with your number.  In the note, simply explain it might be a good idea for the two of you to talk to one another.  It concerns her relationship with your husband, which she may or may not be aware of.  If she calls quickly thereafter, then perhaps she doesn't know.  If she does know, all she'll do is alert your husband giving him time to prepare for you, which takes away your advantage altogether - hence, don't contact her until after you've dealt with him.

There is one final scenario that could occur and although it is unlikely, it's within the realm of possibility.  It could be that he's done nothing with this woman and perhaps they're engaged in an emotional affair at this point and perhaps, not even that but literally just friends.  Men will often times lie for no reason other than they don't want to deal with what their perceived conversation will be with you if they tell the truth.  I think it's hardwired into them somehow.  I had to work for years to break my husband of it and ultimately, it took God (not me) to do that.  ;-)

If he's, in fact, innocent, but just an insensitive oaf, then sobeit.  You'll still have to deal with all of that.  Perhaps, in your calm approach, he'll simply tell you nothing's going on and your automatic response to him should be that if that is the case (but only if you really believe he's sincere at this point - that can be tricky - watch for overly emotional responses either way...toooooo sweet to you and ready to make you feel better about it OR tooooo angry and defensive), then he should know that his actions are causing you to doubt him.  There are obvious problems and it's important you work on them together.

Now, here's the big one...you have take some responsibility here, too.  If he feels like all this is is an attack on how he sucks as a husband, you'll get nowhere.  You'll need to be ready to communicate with him openly, but lovingly.  Accept that perhaps both of you simply need to learn to read one another a little better.  Offer up something you could do for him first or ask him what you could do - don't criticize, just listen.  Then, you do the same.  Soon, hopefully, you'll be in a deep discussion.  It takes work even from there.  Choosing to fix things takes months and sometimes, years.  You must be patient and willing to work on it even on the days it seems it's only you. 

My final advice is to pray.  You may not want to, but do so.  Let God heal both of you and give you strength.  Allow Him to guide your thoughts and words.  Allow Him in to help you with this difficult time.  Pray and be willing to listen to His guidance - don't just look for the solution YOU want, but rather, be open to what He wants for you.  Trust me, it can become the biggest blessing of your life.

I wish you much luck.  Sorry it's a lot to take in, but I truly hope you are able to get through this and heal your marriage. 

Be blessed always.