James - my replies to your questions:
How did going down this path affect your marriage/relationships? My wife is ... concerned. It's really hard to talk to anyone that isn't into this as saying things like, 'It feels like dying before dying' and 'James is slowly disappearing' really freaks them out. She's a bit like, 'Where does that leave me in all of this?'. I don't have a clear answer which is enough for me to want to put on the brakes and never sit again?
My wife was not thrilled that I took to meditation. She saw it as
time away from family. It took some number of months to do, but I slowly introduced her to the notion that I was meditating to
improve my family relationships. I gave her some of my books to read. We talked about the purpose and the process of meditation, and I chose times to sit that were unobtrusive to our family setting and our usual time together. And thus the hurdle was overcome.
I did not share and I'm still careful about what I do share, in regard to the deeper realizations that practice brings. You can't ask someone who loves you to grok what you're doing by telling them you're trying to dissolve your ego, kill your "self," and the like.
Don't do that! You'll be freaking her out.

What can you do to 'stay on the rails' and not give it all up?
I was so deeply into figuring out how my mind works and solving my life-long "being out of sync with experience" issue that I never actually had this problem. Curiosity was my biggest motivator.
Why should I keep doing this? I am getting the feeling of losing the purpose of it all. Right now it seems more detrimental than anything. I do understand that is part of the path but disidentifying with the self on paper isn't all too appealing to the ego that has to die to get there.
See my previous answer. Also, it turns out that while the process can be painful at times the fruit of meditation is wonderful, liberating, and extremely useful in all parts of my life.