From all I know, being highly introspective is not so much a choice, more a trait in some people. Then it's just something that goes on in one's mind regularly and, at least for me, it is very interesting and rewarding. I think the kind of introspection when someone is very prone to dwelling on negative things and insecurities, a lot of self-flagellation, can be stressful, but that's not what I do personally. It's just one of the most interesting things to understand psychological mechanisms and the brain - that's why I also do that as a job. I don't think it has to compete with constructive action and be either or. Of course if it is, that's a problem. But I personally don't think I would have been able to stay sober without understanding my triggers, what I needed to deal with differently, and how.
I've never been in a relationship with very narcissistic people but from the encounters I had with some that definitely qualified for NPD, I learned I was a very bad combo with them. I don't tolerate manipulation from anyone. A few times I had to deal with such people in professional contexts, it didn't end well, they can really bring out my ugly side. One of them was a psychotherapist I saw for a while and we ended up in some of the worst conflicts and fights I've ever had with anyone. I should have just left without fighting with him as it was completely pointless, his behavior was also very chaotic and self-centered, that "therapy" was everything but healing. He also had amazingly little self-awareness for a therapist that I could not even believe at the time and thought he was consciously manipulative, but what I learned from HG Tudor later confirmed that many of these guys are not that smart at all and they don't truly know what they are doing and are more just acting in the moment for the instant gratification they get from their "supply" (HG calls that "fuel"). Hard to understand for someone who is not that way but it is very similar to an addiction, except that most of them never want to quit.