Hi, trikemom, welcome to SI, the best club no one wants to join.
I am so sorry you find yourself here, like you, we betrayed spouses were blindedsided by the person we put our faith and trust in.
I will echo the above posters that you should tell the other spouse. He has a right to know he is living a lie, and his must get tested for STDs (as should you).
This emotional roller coaster is extremely painful and exhausting, but you will get through it with time.
Your husband needs to go NC (no contact) with this woman immediately. No phone, emails, IMs, texts, nothing, nada. Totally transparent with all of his media, phone records, social media, etc. Accountability for his whereabouts at all times. Individual counseling to help him figure out what he was thinking and how he can fix this.
Counseling for you would also be a good idea.
Two books I highly recommend are Not Just Friends and How to Help Your Spouse Heal from an Affair.
Read the Healing Library. Chock full of information, and information is power.
Keep in mind that he owns his OWN actions. It has nothing to do with you, don't ever allow him to make excuses for his behavior.