In my past, I dated a non-Jew; what is the rectification?

Greetings, The way to repair the blemish caused by this forbidden relationship is through complete teshuvah (repentance), which includes three stages: a. Regret for the past. b. Verbal confession as outlined below. c. Commitment for the future not to return to this sin ever again (Rambam, Laws of Repentance, Chapter 2, Halacha 2). Verbal confession means you should say, "Please Hashem, I have sinned, transgressed, and rebelled before You and did such and such. Now I regret and am ashamed of my actions, and I will never return to this matter" (Rambam, ibid., Chapter 1, Halacha 1). If you do as mentioned, the blemish is completely erased, as if it never was, and your sin and transgression are atoned for. However, you must know that you are prohibited from marrying a kohen, as is the law for anyone who has had relations with a non-Jew (Shulchan Aruch Even HaEzer, Siman 6, Paragraph 8). Best of luck, Hillel Meirs