Dear …
Please accept this letter as my notice of resignation, effective __(date)__. Although I thoroughly enjoyed my position at __(name of firm)__, I feel the time is right for me to close this door, enabling a new door to open. I am grateful for the experience and for the relationships that I have made along the way.
Thank you for the opportunity and all the flexibility afforded to me over the past __(#)__ years. I wish __(name of firm) __ continued success in the years to come.
Sincerely,