For me, One Day at a Time has several meanings.
You know those people who think that they won't be happy/successful until "XYZ" happens? Like, when my financial difficulties are fixed, I will be happy." "When I can afford a boat and a horse trailer then I will feel better." "When my husband gets well I can be content." All of my thoughts of happiness were FUTURE BASED.
What I am learning is that my happiness, my wellness, my recovery cannot come later. It can only come today. And every day. Each morning when I wake up is a new opportunity to make the choice to be well, or to make the choice to be miserable waiting on a future that may or may not ever get here. My children will change from today to tomorrow. I can't worry about how they'll be when they're 10 or 15 or 50. I need to pay attention to how they are NOW. My One Day at a Time means to me that enjoying my journey is my standard of success, not any particular destination.
The other thing One Day at a Time means is that some things are too painful to contemplate. Its like quitting smoking -- I couldn't think about how I'd feel if I never smoked again. I had to think about not smoking for the next five minutes. Once I made it through that five minutes, I'd think about the next. Soon I was taking it a whole day at a time. Today, I will not smoke. Tomorrow is too far away to consider. So it is with my codependent relationship. Today I will work on not controlling my husband's life. Just today. Tomorrow is not today - yet.