The speaker describes giving themselves to someone in three stages, and each one ends in hurt. First they open their chest and show their grief, but the other person turns away, so the heart gets locked up. Then they open their mind, and the person leaves traces of themselves in it. That part is locked too, with the key thrown away. Finally the speaker lets the person all the way in, through the "gates," and calls it their biggest regret.
The twist is that the locks now work against the speaker. The person is stuck inside, and so is the memory of them. The speaker is angry and tired, pleading "Get out of my head," and compares the person to a parasite who stays and causes trouble without even knowing it. The last lines are bitter: it "must be nice to run free" in a garden that belongs to the speaker. The mood is raw, resentful and claustrophobic. It captures how someone can keep living in your thoughts long after they've stopped caring.