Living "one day at a time" really boils down to being oseik in today's business today, and tomorrow's business tomorrow.
Taking baby steps in turning our will and our lives over to His care. In other words, to start trusting that He'll make today's efforts work out right - so we do not need to feel like we have to jump ahead and worry about tomorrow, next year, or next decade. That unknown and untrustworthy future is where many of our self-centered and childish fears live and breathe, you know.
And we slowly begin to let go of the pains of our past and trust that He'll make it right, too. That He will help us do what we can to correct things, and help us accept our powerlessness over what we cannot. Like the serenity prayer talks about.
These things are probably among the most basic to true yiddishkeit. Just basic simple trust in Hashem in our real lives - in our marriages (or lack thereof), our schoolwork, relationships, growth, and health - real life, rather than just a sefer or a sugya. We can grow tremendously in learning and practice of Torah and mitzvos without this, of course. And normal people can get by...many probably do, nebach. But for an addict, too many 'alarms' go off and life becomes too unbearable. We are just too screwed up, I guess, so we need to learn to live in the present, otherwise we cannot really live, at all.
Not a very bad fate, really.
To me, what sets it all apart from the almost useless faith I had before, was that when I heard about bitachon, it was all about the extremes: Either I went all the way and trusted Him fully, or not. Recovery showed me that real bitachon grows out of the little, unglamorous, insignificant-appearing things, and comes in tiny, little steps. To me, that used to appear unimportant - not what the sforim are really talking about - or maybe even as apikorsus, or at least hypocrisy. B"H the truth is that useful emunah and bitachon are only learned through real life action, that is, from taking steps - and not from s'forim and 'step-study'.