How I would describe it would depend largely on the type of character it is. Like George said, I’d probably start with disbelief or denial. Like something like that would take a while to sink in, if it ever sinks in at all. (which it might not if they have no memory of it) I would play with a lot of confusion, disbelief, numbness, not wanting to hear what they’re being told. Maybe anger at people telling them they would do such a thing. Maybe anger at themselves for not remembering or if they do remember, obviously for that. Over the next few hours or days or weeks, lashing out at small things, pulling away from people they love, acting cold, or uninterested, or even actively hostile. Or you could go another way and make them totally weepy and needy or even catatonic with guilt.
I think with that level of guilt, a person could easily and quickly become pretty self destructive. On a bit of a longer term, drug use and/or promiscuity, shift in personality towards things more reckless and spontaneous, even death defying. That kinda thing. Sounds like you’re gonna have fun with it!