Issue not resolved - must resign
Please accept this letter as resignation of my position as __(name of position)__, effective __(date)__. Only after long and careful consideration of all factors did I finally make this tough decision to resign. I regret leaving friends here but I feel this change will be beneficial to my long-term career goals.
I wish we could have come up with an agreeable solution to the struggles I have encountered with __(name of company/supervisor/co-worker)__. It was my desire to work and grow with this company for many years to come. I have enjoyed my job with __(name of company)__ for the most part, and would have liked to continue working here. Perhaps it was not meant to be.
The recent job offer I have received will allow me to grow in the areas of my career that I feel are vital to my success and personal happiness. I am giving __ weeks notice, providing you with the opportunity to find a suitable replacement. I am more than willing to provide training and orientation to the new person.
It has been a pleasurable learning experience working as part of your team, here at __(name of company)__, and I wish great success to you and all of my co-workers.