Dear Mr./Ms. Bossname
It is with mixed emotions that I tender my resignation as (position) from (company), effective (date).
I have struggled with this decision, and ultimately it is for the best that I depart to (focus on my health, go back to school, start a new career path, etc.). I have appreciated the excellent management, coworkers, and opportunities at this company.
Thank you and I wish you all the best.