Broken trust is a single sentence about a friendship ending, and the way it's put together says a lot. The speaker looks back at the day a friend walked away because of one mistake. What breaks isn't only the friendship itself. It's the speaker's belief in "eternal friendship," the idea that real friends stay no matter what.
The feeling underneath is hurt, mixed with disbelief. The word "single" matters here: the speaker seems to feel the punishment was far bigger than the mistake. There's no shouting or anger in the lines, though. The tone is quiet and flat, almost like someone stating a hard fact they've had to accept.
The poem has no punctuation and no break, so it reads as one breath, as if the speaker is still stuck on that moment. The mood is sad and a little disillusioned. Losing the friend also means losing a comforting belief, and that is what makes it sting.