After presumably screwing up their relationship with an unspecified person (could be anyone from a friend to a family member), they're sitting up at night the next day thinking about how they could have avoided the situation or made things better, until they eventually accept time goes on - closing by saying "I'm stuck in the past!" to acknowledge what it is they're really doing.
The title is also likely a reference to the character "Sharptooth" from "The Land Before Time" series. It is entirely possible that the character "Sharptooth's" reckless behaviour is being compared to the narrator's own within this song.
Pretty simple idea they've got going here.
After presumably screwing up their relationship with an unspecified person (could be anyone from a friend to a family member), they're sitting up at night the next day thinking about how they could have avoided the situation or made things better, until they eventually accept time goes on - closing by saying "I'm stuck in the past!" to acknowledge what it is they're really doing.
The title is also likely a reference to the character "Sharptooth" from "The Land Before Time" series. It is entirely possible that the character "Sharptooth's" reckless behaviour is being compared to the narrator's own within this song.