Sorry you are here. Some thoughts.
Realize that the women you married and loved is gone. She has been replaced with a lying cheat. She may look the same, but its not the person you used to know.
If you live in a state or country that has "at fault" divorce in cases of infidelity, then get a PI for sure. You will want 3rd party collected evidence that cannot be explained away. If it were me I would go with a PI anyway.
Over on SI there was a poster named Spaceghost0007 who did just that. The PI found all the evidence. He confronted with the shock and awe approach. And she never got the upper-hand. Unlike so many other BH, he did not get sucked into endless arguments and trickle truth. Many times the WW will only admit to what you have evidence of. Decisive action is always the best approach to get out of infidelity.
If you want to read his story: SurvivingInfidelity.com - Thought we had a good marriage
Hemming and hawing, doing the "Pick Me" dance, begging her to give up the OM and work on the marriage. It never works. Only taking control of the situation works.
So play the long game here. Do not try and win a battle, win the war. Resist the urge to confront until you have all of your ducks in a row. Evidence. Divorce papers. Contact info for the OM spouse if there is one. Then when you have prepared you pick the day and time. Have her served papers at work. Inform family members. And go dark on her for a few hours. Do not answer her calls or texts. When she gets home tell her her stuff in in the guest room.
She will most likely respond in one of 3 ways:
1. Cold indifference, the whole "I have not been in love with you for years" line. (Not likely unless this is an exit affiar)
2. Rage and anger that you spoiled her fun and ruined her reputation with the family. (Possible, but remember she brought this upon herself)
3. Tears, crying, I'm So Sorry, I was never going to leave you, I never stopped loving you. I never meant to hurt you. (Most common response)
If she opts for #3 it will look and sound really sincere. Its not. No one is remorseful that quickly. They are sorry they got caught. It is just her covering her ass and trying to avoid the consequences of her actions. Stay strong and prepare yourself emotionally. Stand your ground.