The arizal says that it is a big sin to have sex with a non jew. If someone had sex with one how can he repent for that?
Rav Moshe Stern, the Debreciner Rav, has a responsum to a group of people who had repeated intercourse with various non-Jewish women. After dealing with a number of other issues (such as if wearing a condom during intercourse mitigates the prohibition in some way, and which particular transgressions were involved in that case), he concludes (Be'er Moshe 4:141-142, my summary/loose translation below):
על כן, ירבו בבכי ובחרטת הלב בלב שלם ויקבלו תעניות כפי כחם וה' הטוב יסלח להם. והעיקר הוא חרטת הלב והחלטה נחושה לבל ישובו חס ושלום אל כסלם...
Therefore, they should cry a lot and regret their actions sincerely, and they should accept fasts according to their strength, and Hashem the good should forgive them. The focal point should be the internal regret, and a concrete decision not to return, Chas Veshalom, to their silliness.
Reb Moshe has about 3 teshuvos on this sort of thing, I think he is discussing teshuvah for homosexuality as far as I remember.
The issue with having sex with a non-Jew is more a hapharas bris than an issur de'oraysah, in fact the rishonim are hard put to pinpoint exactly what the issur should be.
I.e. you have violated the integrity of the Jewish people.
The gra in his peirush on the safra ditzneusah says the bris hamaor is keneged the bris ha'lashon. I.e. the tikkun al pi kabalah for hapharas bris is to do mitzvos involving the mouth, such as learning Torah and not speaking lashon hara. Which is approximately what Reb Moshe says.