The prohibition of sexual relations with a non-Jewish woman(man) is one of the harshest transgressions listed in the Holy Torah.
A Jewish woman who has had sexual relations with a non-Jewish man, or a Jewish man who has had sexual relations with a non-Jewish woman, may suffer harsh tribulations in this world and in the World to Come. This transgression can completely shut down a person’s good fortune, bring on illness, poverty and anxiety, and various other disruptive situations, G-d forbid. 

The greatest catastrophes experienced by the Jewish people have begun with this prohibition, particularly assimilation with non-Jewish nations.
The Torah clearly writes: “And I shall differentiate you to be a people for me.” G-d separated us from the other nations of the world as far as our sanctity to Him.

The prohibition of marriage to a non-Jewish woman 

The Holy Torah, in Devarim (Deut.) 7:3-4 presents the prohibition’s details:

“You shall not give your daughter to his son nor shall you take his daughter for your son; for he will turn your son astray from following me and they will worship other gods, and G-d shall be angered by you and destroy you hastily.”

The RAMBAM, interpreting the Torah, warns Jewish men not to have relations with non-Jewish women because this prohibition will eventually lead the man to wanting to marry her.
Additionally, we find in the Torah just how severe this transgression is in the episode with Balak.
Bilam in his wickedness tries to curse the Children of Israel but instead, a blessing comes from his mouth, as we know. In retort Balak says: “But I hired you to curse them so why do you bless them?”
Bilam answers: “Let me give you advice. ‘The G-d of Israel hates plotting. But place prostitutes in the market and see how they get destroyed.”
Our Sages explain: Rabbi Levi said: Come and see how much harsher the punishment of forbidden sexual partners is than the sin of the Golden Calf; 3,000 died for the sin of the calf whereas for the sin involving the daughters of Moab 24,000 died, showing how harsh and severe the sin of the daughters of Moab and the prohibition of non-Jewish women is in G-d’s view.

The transgression of relations with a non-Jewish woman(man) according to Kabbalah 

If we are to understand just how severe this transgression is and what its consequences are, we can read the work of the Arizal in “The Gate of the Holy Spirit.” This is what it says:
“One who sleeps with a non-Jewish woman, Rabbi Haim Vital says, ‘I did not receive the number of fasts from my great teacher, I only received that such a one reincarnates as a dog /{if he did not repent}.”
Most transgressions can be rectified with several fasts, but the transgression of relations with a non-Jewish woman (man) is so severe that fasting is not effective.

What should be done about this situation?

What we need to know and internalize is that after reading the above, the transgression of relations with a non-Jewish woman is so extremely harsh that it is related to with the term “Wise is one who sees the future consequence” and all effort must be made to rectify this transgression.
The Tikkun for the transgression of relations with a non-Jewish woman is based on true, full repentance, and the order of Tikkun set by the Ben Ish Chai and the Arizal, as follows:
Vocalizing, Fasting and Charitable funding.

  There is a special order of study to rectify this transgression, which includes study of the Torah, the Six Books of Mishnah, and the Zohar.
  A person who finds learning difficult can donate NIS. 500 and be assisted by a married Yeshiva student who will carry out the learning program on the individual’s behalf. 
  Here at Menachem Tziyon we can assist through a married Yeshiva student who will become your Shaliakh (representative) for this order of study and will also conduct the confessions with special prayers to rectify the transgression.
  Fasting. Fasting for a night and a day.   A minyan of married Yeshiva students will be recruited to fast for the entire day in order to rectify the transgression.
  Charitable funding. The order of redemption via charity is 216 *18 = NIS. 3,888.

  May it be G-d’s will to bestow purity on us from on High as is said: “And I cast pure water upon you and you were purified.”