When a job applicant realizes after an interview that he or she must withdraw from consideration, this letter can be used to explain the factors leading to the decision and to offer an apology for any inconvenience.
Dear Mr./Ms. Bossname,
Thank you so much for meeting with me (day/date) regarding the position of (position). Unfortunately, I must withdraw from consideration.
After we spoke, I realized (factor). Clearly, this would not be a good match.
It was wonderful to talk with you and I apologize for taking your time in this manner.