he didn't want to tell me because he didn't want me to think he was keeping track of her.
...really him saying-- I don't like to see the pain I have caused, don't like to face the choices I have made....so I wont.
You are not overreacting. The whole thought that a WS knows what is best for a BS to know is a throw back to how they operated while in their affair. It must completely stop for the new pains to stop...
How did you find out?
God be with us all.
p.s. We are all human, we will stumble as we learn to R. We will stumble in our marriages in the future. So on the flip side of my statements above....it is unhealthy to expect perfection from yourself or your husband. This is a good opportunity to change the rules of your situation. No fueling up at the convenience store, no interaction with this store at all. etc.
Find a way to admit the start of how this fact was discovered was less-then ideal, but move into how your finish can be healthy and marriage affirming.