There's an interesting Gemara in Nidda (24b):
Aba Shaul says, I used to be a grave-digger. One day, a cave opened up under me and I stood in a (huge) eye-socket of a skeleton until my nose. When I went back, they told me it was the eye of Avshalom. And lest you'll say that Aba Shaul was short, he was the tallest in his generation. And Rav Tarfon, who was the tallest in his generation, came only to Aba Shaul's shoulders. And Rav Meir, who was the tallest in his generation, came only to Rav Tarfon's shoulders.... etc....
The Maharsha writes that this Gemara is a Mashal. Aba Shaul is saying that he was once sunk in impurity. It reached a point where he had fallen in until his nose - meaning, he could hardly breath a breath without impurity. Then he says "when I went back" - meaning "when I did Teshuvah", they told me it was the eye-socket of Avshalom (son of David). Avshalom had "huge eyes" in a meta-physical sense. He desired to take over his father's entire kingdom and take his father's Pilagshim (concubines) as well. He had eyes that desired to swallow everything that was not his. Aba Shaul is saying that once he did Teshuvah, he was able to recognize that the impurity he had sunk into was a result of this "eye of Avshalom", which means "the desire to look at and swallow all that was not his". And the Gemara continues, lest you say, Aba Shaul must have been a low-life to have eyes that desired everything, no! he was such a great person, that he became the tallest (read: greatest) in his generation!
We can learn from this Gemara two important lessons.
1. The source of impurity is the eyes. The eyes see and desire that which does not belong to them, and through this, one can sink so low until he can hardly breath without impurity.
2. When a person who has these struggles does a true Teshuvah, he can reach the greatest heights of spirituality.
So never say, "I have sunk too low. For me it is too late". Aba Shaul was there, and he became the greatest in his generation!