Even though we don't know that the speaker is aware of how quickly her candle will burn out yet, by the first line we're already thinking that there might be problems in the making. After all, candles don't last forever. And if they're lit on both ends, they last even less time.
It will not last the night (2)
Here's where the theme of transience gets hammered home in a big way. Our speaker doesn't beat around the bush. No, she just announces for everyone (including herself) that her brilliant light is going to go out… even before the time when it's supposed to be lighting the night. We get the sense that it's going out too soon—after all, night without candles is a pretty dark, scary affair.