Hi BF,
What you describe sounds perfectly normal to me. How long have you been married and how long were you in a relationship overall? I'd say that the end of any relationship, no matter how difficult will cause this sadness to occur. There would be more of a concern on my own part if I didn't feel any sadness to be honest.
Maybe it's the sadness I truly feel for her.
This is probably true. Have you also considered that it would be natural to feel sadness for yourself? After all, this is a loss. You may not consider it a loss of the difficult stuff, but at some point things must have been good and that is lost if you've reached this point. So a sadness about what could have been but wasn't would also be reasonable to expect. Sounds to me like you're going through the expected process that any divorce or breakup would cause. It's OK to feel this way.  :)on't judge yourself.
Love and light x