Lori, please don't misunderstand.
You "sound" as if you have regrets about leaving, you spend too much time explaining why!!
To be honest, whatever you've been told about cold hard termination letters, is bullpiss........
You have "CHOSEN" not to subscribe to their services any longer, as of the dates given, and have NO NEED to tell them a "story"...that is for "clingers".....sorry, that's the way I "read" it!!
Basically, set the title, let them know you're leaving, let them "know" that could, AND might consider re-joining at some later date, when THEIR businesses AND practices, are MORE in tune with YOURS, and YOUR budget allotment, and leave it at that!!
Send the letters, REGISTERED, and "signature required", request an immediate response "acknowledging" the letter, then FAX the SAME letter to membership, THEN, wait ONE day, and "courier" your" resignation to membership, overnight...Mark it "Important Document Enclosed" send it to VP of your area..forget the field reps!!
Then, just DO IT....git 'er done!