[Cron]

Ask her flat out (respectfully) if she is cheating on you.

If you don't have trust in a relatioship you have nothing. After she gives you an answer you need to decide if both of you are willing to work on rescuing the relationship( rebuilding trust, communication, etc ).

When you ask her you might want to not mention that you know. Could be a good test towards seeing if you can trust her.

You are going to have to rely on your judgement if she is likely to cheat on you ever again.

You are entitled to feel upset, but whatever you do, do not catasrophize this situation in your mind. People break up every day, good people are cheated on every day, people have the love of their lives die everyday. They are ordinary people who hurt like hell when it happens, but they recover and move on. You can and will too.

No matter what happens you will probably want to find a new job as at least you will feel incredibly uncomfortable at this job and may put your career at risk depending on how other people handle you asking/finding out.

Again, it IS a big deal, you are entitled to feel devastated, but it happens to people and they recover. Remember that and it will help you get through it quicker.