Brooklynn's speaker describes a heart that feels less like an organ and more like a machine breaking apart. Its "stray pieces" chase the speaker, and running doesn't help, since the damage is inside and goes wherever they go. The image of being "derailed" adds to this sense of a life knocked off its track.
Then comes a breaking point: the chest "bursts open" and big, heavy feelings spill out as words. But the release is messy. The speaker stutters and seems caught "not quite in but not quite there," unable to say things clearly or find the right balance between holding back and speaking out. Judging by the ending, the attempt fails to bring relief.
The last line is the saddest. Instead of being helped, the speaker welds the heart back together, closing themselves up again. The mood is tense, tired, and guarded, and it suggests that hiding pain can feel safer than trying to share it badly.