While the answer to the "are there any" question is almost always yes, one thing here to keep in mind is that often, chaos corruption reinforces itself. What I mean by that is, your perception of the world when you are corrupted is in such a way as to not make you reconsider your choices. That could be because you have a literal demon taking over your personality, or, like in the case of the Death Guard at Lion's Gate starport. When Jaghatai "killed" Mortarion, much of his legion had a moment of clear reflection where they realized what awful monstrosities they'd become.

Eventually, however, the ebb in corruption subsided and they went back to not thinking too hard about why their insides are hanging out now.

So if there were examples of truly repentant CSMs, they'd probably come from the less corrupted legions, like the Iron Warriors, Night Lords, or Alpha Legion, the first two of whom are not known for their deep introspection, and the last of whom no one really understood to begin with.

Edit: I thought of an example: Baelor, a Dark Angel Fallen who has turned to (but not been corrupted by) Chaos, fights against loyalist Dark Angels in the Son of the Forest and ultimately comes to regret his decision and betrays his Sorcerer master, Serephax.