But I feel like I can't keep looking over my shoulder anymore I just need to be completely rid of this cult.
Since you indicate that you successfully faded, has there been anything that has actually warranted looking over your shoulder?
As you said 'But I feel...', you are referring to personal feelings within you. If you perceive yourself as having already left, why do you think sending a resignation letter will change how you feel?
If you think that sending a letter will change your thinking on how you will feel, why can't you modify your thinking so as to be at peace, with or without sending a letter?