@coopey Back when I was in my 20s I learned from an older co-worker: keep that letter. Re-read it when you need to (it helps), and edit as appropriate. Some day you will have a chance to use it.

And if things ever get so bad that you are willing to resign without a new job in sight, you'll have a thoughtful, well-written letter instead of something dashed off in haste and anger.

Best wishes for the search.