You are correct to think she isn’t “wrong” to feel this way, her experience is valid and she has a right to feel however she feels. It’s her response and actions that I think are the problem here. If I found a sex tape of my spouse I might be upset, but if they immediately deleted the video and if it wasn’t part of a pattern of infidelity I don’t think it would be a big deal for me.

I honestly don’t think you fucked up here, at least given the information that we have. If having that video was a mistake then you should have deleted it… which you did immediately as soon as you were reminded of the video existing. People forget things all the time, and IMO your response to becoming aware of it again sounds like it was perfect.