With maturity we should be moving from irrational to rational, emotional to logic and rational though, in understanding, intent, speech and actions.
Just for today is a concept that allows us to get through rough bits of life. We can labor hard through short term issues for a bit. I can do, just for today, what I could not bear for a lifetime. (if in doubt of which one, try a furry animal)
or someone else take:
Living just for today relieves the burden of the past and the fear of the future. We learned to take whatever actions are necessary and to leave the results in the hands of fate, nature, other people. The theists puts in god, the non-theists, nature.
But is this not living in the present, without hope for or fear of the future? (stoic description) The down side of hope maybe creation of a delusion, that delusion being that things will be better in the future. They may be different, and not better. We are better to deal with reality verses living with a delusion. Any god pusher thinks that it is better to live in a delusion with irrational hope, ... is better than facing reality.
Now to those who through (or should that be throw, as to pitch) in a what if it never gets better? Well, we have a choice, live or leave. We always have that choice, and if it is logical, perhaps to return ourselves to where we came from, may be the best answer, while we are able and the choice is up to us.
Just for today, I will try to understand the logic. But what do I know?