There is no shortage of Kabbalistic sources stressing the severity of succumbing to sexual temptation. Here are some more.

Dowload PDFs of some amazing books:

In-depth discussions in English.
GUARDING_THE_EYES-E.pdf

44-page book in Hebrew, compiling sources:
SCHAR_ONESH_SHEMIRAT_ENAIM-H.pdf

TIKKUN_HABRIT_BERESHIT-H.pdf 419-page book in Hebrew!
ginzay-hamelech.pdf From R Yaakov Abuchatzeira's book Ginzei Hamelech.
kedushas-habris.pdf - By the author of Chak L'Yisrael, for the days of Shovevim .
rashbi-bris2.pdf Another Hebrew book for download.
TIKKUN48-H.pdf-  kabbalistic incantations by the Ar'i Hakadosh. If ou don't know how to use this, don't try.

Sources in English, compiled by an anonymous Tzaddik.
 GUARDING_BRIT-E.pdf

A selection of books in English and Hebrew that deal with rectifying the brit in depth.

Lest you think the issue is only for kabblists, the Kitzur Shulchan Aruch (by Shlomo Ganzfried (1804 to 1886), one of the most important summaries of Jewish law, dedicates a full chapter to its severity, including rectifications for baalei tshuva (repentants). The text here is in Hebrew only.