Here, the "monster" seems to stand for a powerful emotion that the speaker can't manage. The poem never says exactly which feeling it is, which makes it easy for readers to fill in their own: anger, anxiety, sadness, or something else. What matters is the sense of losing control. The monster arrives "without your permission" and takes over, and the speaker is left helpless while it does.
The tone is flat and tired rather than dramatic. The short, plain lines feel like someone who has been through this many times and doesn't have the energy to dress it up. There's a small flicker of hope in "it may eventually go away," but the next line takes it back: "someday it will return." The closing phrase, "a vicious cycle," lands like a shrug of defeat. The mood is weary and resigned, with a quiet fear underneath that relief is only temporary.