A heartfelt resignation letter ideally reflects on the time you spent at a company. If you left on good terms, also consider sending a personal thank you note to your boss following a resignation letter. It will help you close one chapter of your life while building bridges for your future.
No one expect you to be the world's greatest writer when you write a Thank You letter. All you need to do is be yourself and tell that person why you are thankful for them. For example if a professor has really instilled a love for mathematics after a class then tell them just that, or if you are finishing with college and you're on a sports.