Surviving infidelity, How can I trust her again?

This story came in from @pete434 in our infidelity support group.

I'm not sure who is the bigger fool. For the longest time I was led into believing my wife had a highly developed moral compass all to learn she cheating on me recently. It took an hour of me grilling her a few nights ago for her to cave in. Apparently her sexcapade happened 2 months ago when I was tending our child.

You see, I'm a stay at home dad. I have been unemployed for now going on 9 months. My wife is runs an insurance type business and often times meets her clients outside of the house while I tend to our son and dog. This particular evening she decided to meet with a client in the early evening. I thought nothing of it at the time, but as the evening wore on, I began to have a sick feeling in my stomach. When she got home later that night, I noticed she immediately went to the restroom and used mouthwash. She didn't even greet me when she came home like she normally does.

Anyway, long story short it turns out she had unprotected oral sex with her client. After learning about this I feel gutted and destroyed mentally. I can't even look at her in the same light. It's been several weeks and I have barely spoken to her. I feel like less of a man that someone else got to enjoy my wife in a way that only her husband should. What should I do?  We have a child and I don't know how I can fix this at all. She swears it only happened once, but I feel otherwise.  I'm so insecure about this that I check her phone every opportunity I have. I even followed her when she went to the store to make sure she wasn't orally entertaining herself! I'm consumed with anger and rage. Please help!