The gemara (Chagiga 15) relates the tragic story of Acheir, the teacher of Rabbi Meir, who went "off the derech." Rabbi Meir asked his teacher why he did not repent, to which Acheir replied that there was no hope for him, as he had heard the Heavenly voice of a bas kol declare that Hashem's words, "Shuvu banim shovavim," Hashem's encouragement to repent, applies to all except Acheir. It was prophetically decreed that he could not do tshuvah! Nonetheless, the gemara continues that Rabbi Meir on other occasions tried to get his teacher to return, all to no avail. Tos. cites the conclusion of the story from the Yerushalmi -- R' Meir visited his teacher on his deathbed and once again asked him to repent. Acheir cried, and at that moment his soul departed. Rabbi Meir interpreted that cry as an act of tshuvah by his teacher in his final moments.
Whether or not one accepts the conclusion that Acheir indeed did do tshuvah, the story begs the question of what Rabbi Meir thought he would accomplish by his repeated entreaties to Acheir to repent. If the bas kol declared repentance impossible, why did Rabbi Meir still hold out hope for his teacher?
To relinquish one's past attitudes, direction, deeds, is exceedingly difficult. How is tshuvah ever possible? The answer is that we have the greatest possible help in our efforts. Hashem doesn't just command us to do tshuvah -- he takes the first step himself towards restoring a relationship with us and guiding us back to the right path. Without that assistance tshuvah is still possible, but it requires so much greater an effort.
R' Itzele Peterberger (Kochvei Ohr #6) writes that it was this encouraging voice of, "Shuvu banim," Hashem's extended helping hand, which the bas kol declared no longer accessible to Acheir. Yet, as Rabbi Meir realized, the door to return was still open. It would require commitment and effort and be a difficult road back, but it was a road that Rabbi Meir was willing to stand by his teacher in support should he choose to follow it.
We are not in Acheir's position. "Dirshu Hashem b'himatzo," this week Hashem inspires and encourages our teshuvah, extending his hand to help us. All it takes is a little effort on our part.