Each situation is different and the aftermath of an action can be very different.I understand how this is eating you. In most cases confrontation leads to separation.BBe prepared and willing to do that. Confronting just to say you know is not enough. You need to state you will not be in a R with her if there is another person. You need to know your boundary and be willing and able to enforce the appropriate consequence if crossed.
Only you can decide your line of action but be aware that confrontation rarely results in an immediate turnaround in cheating spouse. It can be necessary to step up and confront. In fact many will advise it is your only option if you want to save yourself and your M.
It is not your only option but there are no easy options: 1 you could carry on ignoring A and learn to deal with it eating you up. I personally don't see that as a viable long term option bit is OK whilst preparing your next step. 2. Confront. Regain some pride at the risk of ending your M. It is a risk but a M with a third person is not a M worth having. Regardless of the aftermath this does not mean you will not reconcile later. 3. Do not confront but let her go. You live your life as if you do not want anything more to do with her. Basically you distance yourself or drop her without a big explication.
Just remember in all of this that the A is a symptom of a bigger underlying unhappy situation. Ending it does not automatically mean she will recommit to you.
If you cannot live like this, put together a plan . Prepare for the worst, though hope for the best. Regardless of your W, you need to work on you. Without belittling an A (I am not, it is huge) but don't let it consume you to the point that you are not working on you and your goals.
Best wishes
R 25 years M 14 years S11 & S13 Working on it alone since Oct 2014 M in trouble a lot earlier (~2 years) Feb 2016. 1st R chat in a yr. Next R chat Aug'17 Still together