[thanks to Pesah, my chevruta who helped me understand this idea]
When we turn to HaShem, sorry for what we've done, we begin the Teshuvah process. We tell HaShem what we did wrong, and how we have no desire to continue along this path.
Wait a second.
Right about now the Yetzer Hara says to you: "What do you mean you aren't going to sin again? You're telling me honestly that you are now a tzaddik? That's all well and good, but tomorrow will you still have the strength to enforce today's decisions? You're so sure you will never sin again? Maybe you should be more honest with HaShem, He knows what is in your heart anyways, tell Him you'll try not to sin again, or that you don't want to sin again even though you probably will."
What's your answer to the Yetzer Hara? How do you answer such a true and well-reasoned argument?
At this point most of us concede the Yetzer Hara's point and turn to HaShem saying something along the lines of: "HaShem, I'm sorry for what I did, and I don't want to do it again, but I'm only human and I probably will, please forgive me."
I'm sure our honesty is not lost on HaShem, whose infinite mercy is beyond our comprehension. However, what just happened?
We just caved to our Yetzer Hara. It might sound good and true, but if the Yetzer Hara said it, you can be sure at it's root it's a test, a lie, it's just not true. If it's what the Yetzer Hara wants, isn't that enough of a reason to fight it? To say, "No! HaShem, I'm sorry for what I did and I'm NEVER going to do it again EVER!"
Right now you're reading this and basically siding with the yetzer hara, "Well, it's a nice idea, i'd like to be able to really do that, but wouldn't I be lying to myself and to HaShem?"
Rebbe Nachman teaches us that the key to serving HaShem is knowing that we only ever have today. If we are blessed with another day, a tomorrow, when we get there, it becomes today.
We only have today.
Just one day.
In that day we can serve HaShem, or we can (has v'shalom) rebel against His Torah, His Will.
When we say, "I sinned, HaShem, I'm sorry, I'll never do it again." We mean, "All I have is today, and I'm going to make sure I won't do it again today." If we get to tomorrow, we'll see what happens, but right now there is no tomorrow.
The Yetzer Hara is always trying to get you to focus on tomorrow, not only tomorrow but all the tomorrows still ahead of us. And the yesterdays. He says to you: "Yesterday you sinned. Tomorrow you will sin. Why don't you sin today too?"
To this we need to respond, "No." Today I'm not going to sin. Today I will serve HaShem, I'll worry about sinning tomorrow.
The Yetzer Hara wants you to push off your mitzwoth till tomorrow, all we need to do is push off our sins till tomorrow.
So, now when the Yetzer Hara tells you, "But you're going to sin tomorrow," You can agree with him, yeah, but it's always going to be tomorrow. Just so long as I don't sin today.
Now we can all go forward, and do proper whole and complete Teshuvah, and say to HaShem, "I'm sorry I did it, I'm never going to do it again. EVER." And know in our hearts the truth of it.
All we have is today. Today to do mitzwoth. Today to do teshuvah. Let the Yetzer Hara worry about tomorrow.
It's not for nothing that Pirkei Avoth teaches us, "Today to do the work, and tomorrow to receive the reward." Olam HaZeh, this world, is called 'Today.' The world to come, Olam HaBa, is called 'Tomorrow.' We know that tomorrow, in the world to come, there will be no Yetzer Hara. He should worry about tomorrow -- not us.