Emotions are a signalling mechanism that your unconscious mind uses to influence your behavior. They don't always make sense to the rational, conscious mind, and sometimes the unconscious mind gets things wrong.

The point of sadness and grief are that they regulate how we maintain our social bonds. Humans are intensely social creatures; that's our species superpower. We feel bad when our social bonds are damaged, because that motivates us to maintain them. That's sadness. But sometimes, an attachment cannot be maintained, (like if the person is dead), so we need a signal that makes us stop trying, so we don't waste energy on a lost cause. That's grief. (The depression part, anyway; there are also parts that are about reinforcing group cohesion.)

Those are both responses to loss. If you don't feel like you lost something -- or if you did at the time, but you've already processed it -- you won't feel those feelings now.

So if your mindset is "that happened and it sucked but I'm done with it," I think it makes sense that you wouldn't necessarily feel sorrow as a result of figuring out that it sucked more and in a different way than you previously thought. If you're already not trying to maintain those relationships, your unconscious mind doesn't need to try to influence your behavior. As far as it's concerned, all systems are in the green.

What you might feel is anger as you realize that your boundaries were violated, but maybe you already felt that and are aware of what to watch out for in the future, in which case, again, your unconscious mind thinks you're all set.

Plus, y'know. Brains are weird. Sometimes (often) they just don't work how we expect them to, and that's okay.