There was a bachur who was off the derech for ten years, r'l, and then he did teshuvah sheleimah. Someone met him at the kever of the Bnei Yissaschar in Poland, and he was shocked to see this bachur davening with kavanah, like a real yirei Shamayim. They spoke, and the bachur told him what brought him to do teshuvah.
Once, someone told him, "Every night, your parents leave the door of their home unlocked, hoping you will return. They don't want you to find the doors locked in the middle of the night."
"I said, if my parents want me so much to come to return to them, I will not disappoint them."
Hashem also leaves the door of teshuvah open for us. The Tana d'Bei Eliyahu (Rabba 31) writes, "I testify heaven and earth that Hakadosh Baruch Hu is sitting and waiting for Yisrael to return to Him more than a father waits for his son to return." Hashem is waiting for us to return, and Shovavim is a time that we can make that happen.
The Zohar (vol.3 p.126.) says, "Every day, a bas kol calls forth (Yirmiyahu 3:22) שׁוּבוּ בָּנִים שׁוֹבָבִים, 'Return backsliding children.'" The Baal Shem Tov zt'l asks: What is the purpose of the bas kol if no one hears it? The Baal Shem Tov answers that we do hear it. When we have a sudden thought of teshuvah, it comes from the bas kol that our soul hears.
This occurs every day. Suddenly, we get an urge to improve our ways. Where did this urge come from? The Baal Shem Tov says that it is from the bas kol. We hear the call in our minds and souls. During Shovavim, the bas kol is certainly heard in our hearts louder and clearer. The bas kol is calling to us, begging us to return to Hashem, and if we listen to the call, we will improve our ways.
The Pri Megadim (Sefer Hamagid vol.3 p.160) writes that שׁוֹבָב has three translations: (a) rebellion, (b) breaking, and (c) banishing.
This tells us that even if (a) one rebelled against Hashem, (b) broke himself and made himself blemished (in a spiritual sense), and (c) he became banished from Hashem's presence, the bas kol calls out even to him and says שׁוּבוּ בָּנִים שׁוֹבָבִים, that he should return because Hashem is waiting for his teshuvah.
The Pri Megadim adds that it states שׁוֹבָבִים in the plural. This refers to a person who has a history of rebelling and then returning to Hashem and then rebelling again and then returning again. Even so, Hashem doesn't lose hope in him. Hashem awaits his teshuvah.