The speaker describes a person who has become a ghost inside their heart. This someone forced their way in, ignoring the speaker's wish to keep them out, and now they drift around with no exit. The locked doors in the second line suggest the speaker tried to protect themselves, but the feeling was already inside before the defenses mattered.
What makes the poem interesting is the speaker's mixed feelings. They say they want to set the ghost free, and they admit it's for their own sake, not the ghost's. Yet they've "forgotten" where the keys are, which sounds a bit like an excuse. Part of them may not be ready to let go. By the end, the speaker settles into patience, willing to let the ghost wander until it's "destined" to leave.
The mood is quiet, tired, and a little haunted. It isn't dramatic grief but a soft, resigned ache, the kind that comes when you know a feeling has to end but can't make it end on command.