Few lines are as short as these, and few turn on a familiar saying so bluntly. The speaker starts by accusing "they" of lying, and the lie is the old comfort that what doesn't kill you makes you stronger. For this speaker, it doesn't. The grammar even breaks the phrase on purpose: "Don't make you stronger" sounds rough and worn down, as if the speaker's voice has been roughed up too.
Instead of strength, the poem describes damage that spreads quietly. Pain "creeps into your veins," an image of something that gets inside the body and can't easily be pulled out. The last line, "A slow and painful death," shows that surviving isn't the same as being fine. The hurt keeps working long after the event.
The mood is bitter, tired, and a little angry. There's no hope offered, no lesson learned. The speaker seems to be rejecting the idea that suffering is secretly good for us, and the poem leaves that rejection hanging without comfort.