Thanks everybody for your support. It means a lot to me and it does feel good (if humbling, though humbleness can be good, too) to be able to admit I need that support. You can't do this all in your head, alone.
I can't really understand why I'm "scared" to be sober since the two years that I was sober before were great and extremely productive. But I think I ultimately went back to drinking because sobriety was the act of "not" doing something rather than doing something. So this time around I'm trying to build a framework that supports sobriety and replaces drinking... not just by exercising, for example, but by learning to value exercise and health and well-being as a goal in life -- as a purpose that gives life meaning, not just "something you should do." I'm not sure if that makes sense, or if I've fully articulated the difference -- but it seems to me the difference is critical.
I also think this desire for sobriety -- or at least "less drunken partying" -- is tied up with turning 40, and with our natural propensity to turn around and look at our lives at such major milestones and to assess where we're going and we're we've been. And even though I've been a heavy drinker, I have to say that I feel like I've accomplished everything I wanted to achieve when I was 20, and then some... but now I look to the future and I think, what do I want to do for the rest of my life? And in many ways I feel like I have no clue, other than I know I don't want to be drunk every evening for the next 20 years. (And for all I've managed to accomplish, I wonder how much more I could have achieved if I had been sober or just a normal drinker?)
Anyhow, sorry to ramble. It's not a mid-life crisis, or even a crisis at all, but tied up in this desire to not be a drunk, I realize, is a deep longing for meaning and purpose, and a need to figure out what I want to do and achieve in the coming years. And I wonder if anyone else felt their quest for sobriety was tied up in similar questions of purpose as well.