I'm sorry this is happening to you. I know just how it feels.

The letter of resignation is what most people do in order to have a better shot at getting hired in another district. You're lucky in that they told you the real reason behind the decision. That makes it possible to focus on and consider improvement. I was never given a real reason, and when my students liked me and I felt I was doing a good job, that makes things very murky. Maybe they saw the bipolar in me.

Anyhow, you can pull through this. You're welcome to write me via PM to discuss classroom experiences and job trajectories.