On the surface, "A note" is a thank-you message. But the thanks is bitter. The speaker is thanking someone for being absent at the moment they needed help most, and the sting comes from how polite the wording is. Nothing is shouted. The anger is tucked inside good manners.
In the second stanza, the speaker admits what that absence left behind. "Trust me" sounds almost calm, as if they're sharing a hard-won lesson. The person let them down, and the result is a belief that it's safer to expect nothing from anyone. That is a sad kind of lesson, because it affects every other relationship, not only this one.
The mood is cold, dry, and quietly hurt. The short lines and the plain, note-like style make the speaker seem controlled, but the control reads as a wall built after disappointment. Under the irony, you can sense real pain.