Teshuva = Hashem's Will for Me Today
The very phrases "Hayom al l'vovecha" and "asher Anochi metzaveh es'chem Hayom" in kriyas sh'ma are both darshened by chazal to mean that His proclamation here is new - today is the first day we have heard of it! Now, to me, this is precisely living one day at a time. It is an approach to life in kriyas sh'ma itself. It reminds me each time I say it that Hashem is concerned with how I do today. Not yesterday, nor tomorrow. Teshuva is only relevant inasmuch as it affects today's avodah. Hashem asks for one day at a time - should we ask for more?
Still, there are times that the way we understand teshuva doesn't work. It weighs us down in today's avodah. That is when I need to say, "I guess I do not understand teshuvah right now". Instead, I need to do what works right now so that I do Hashem's Will for me now. It usually turns out that what I thought was teshuvah, was really tikkun, a later part of the teshuva process that is poorly understood and often jumped into by all us guilty types to relieve the terrible burden of guilt we carry. Quite idiotic in my case. And as Chovos Hal'vavos says (right at the start of sha'ar hateshuvah), the definition of Teshuvah is behaving correctly right now even though I have screwed up badly in the past. It is not about fixing anything. Hashem fixes, or helps me fix.
In the middle-ages, yidden were motivated to behave better by remembering that malochim or worms would bust their eyeballs if they looked at lust objects; that zera l'vatola was murder of doros, etc... I have not met anyone with a long-term lust problem who actually got better by focusing on that alone. But I have met many who learned how damaged they were, and accepted that they need to treat themselves differently than they thought, because they have an allergy to lust and are hard-wired to feel that it is truly in their best interest to use schmutz or masturbate. (After all, that is exactly what it means when we feel inside that we absolutely need it, right now... isn't it? And we have all felt that way, no? That we needed it... that's why we "fall", R"l.)