Dear Mr./Ms. Bossname,
I wanted to thank you again for all that you have done to help me grow in my position here. I feel as if I'm leaving the company as a different person. When you hired me, I was so green and insecure about my abilities. Thanks to your training, guidance, and encouragement, I am confident in my skills.
You may have done your job too well! After all, I am resigning for another job. But that just makes me all the more grateful. You and the rest of the team were the best part of working for this company. I hope that we will be able to get together sometime soon. I will never forget the adventures we all had at the Widget Convention.
Looking back on the great times here almost makes me regret resigning!