Am I only a "betrayed wife" after I found out? Or was I one all along?

I still, almost five years past D-Day #1, have trouble looking at photographs of "before". My children's babyhoods are now colored with the knowledge that, while I was home changing diapers and mopping up spit, my husband was peeling off panties and swapping spit.
How do I reconcile the past I thought I had with what was going on behind my back? How do I look back at wedding photos/family pictures/celebrations without that happiness-busting sense of "well...we might look happy". The point, of course, being that my reality wasn't...real. Or was it?
It's a sort of philosophical issue. If a tree falls in the forest but you don't know that tree is falling (or more to the point, that your husband is the one chopping it down), does it change the past? Or is your past still your past – memories intact – and it's only your future that's altered?
I'm no philosopher (clearly, given my convoluted example!). But I struggle with my sense of history now. My sense of self is shaky. Am I still the same person I was, even though my life was not what I thought it was?
Am I still the same person even though others' view of me was perhaps colored by information they had...but I didn't?
I want to believe the answer is yes. I am still I. And though my future is certainly altered by the knowledge of my husband's cheating, it doesn't change the mother I was, the wife I was, the daughter/friend/sister/writer I was. I need to stand firmly in my own self. And recognize that another's actions, while they affect me, needn't change me.