The Rambam (Hilchos Teshuva 2, 1) says that, if a person faces the same Nisayon, and overcomes it ONCE, that is Teshuva Gemura. The Rambam also says (Hilchos Teshuva 2, 3) that Teshuva (here he doesn't use the stronger term "Teshuva Gemura") is where Hashem testifies that the person will never return to the sin again. How can this be? It seems that the Rambam is requiring a much higher standard for regular Teshuva than for Teshuva Gemura, which makes no sense!
The answer, as explained in a recent Navi Shiur by Rav Yisroel Reisman, is that there's a huge difference between the Olam Hamachshava and the Olam Hama'aseh. If a person only does Teshuva in his mind, but does not actually overcome a Nisayon, then Hashem has to testify that he will never do the aveira again and, even so, it's only a standard Teshuva. But, if someone actually overcomes a Nisayon, that is considered a Teshuva Gemura - so much so, that if he does the aveira again, it's as if he did it for the first time in his life! (Of course, the person can't plan to do the aveira again, but as long as he accepts upon himself to try to avoid it - and he overcomes the same Nisayon a single time - that is Teshuva Gemura). Imagine, then, how great a person's Teshuva can be if he is Omed b'Nisayon day after day (one day at a time, of course)!
Rav Reisman noted that the second Rambam (Hashem has to testify that he will never do the aveira again) is quoted far more often than the first Rambam and people, therefore, assume that Teshuva is very difficult. That may be true in the Olam Hamachshava. However, in the Olam Hama'aseh, we are given constant opportunities to do a Teshuva Gemura. What a tremendous Chizuk!