[fractallyte]

There are two things here to recognise and differentiate: depression (the clinical kind), and dejection (feeling down about things in general).

With the latter, you can work out a plan of action to gradually lift yourself out of misery. Since you're on HN, you're in a community of like-minded perceptive, intelligent individuals. There's an abundance of advice in previous threads on this subject.

The former is more complicated, and potentially dangerous. It's classified as a 'mental illness'. Professional assistance may be called for; other times one can recognise the effects, realizing that the cause is within rather than without. If a good night's sleep doesn't cure it, seek assistance...

[mrits]

You might want to wait a little longer than one night before you seek assistance. Anyone that has ever ended a relationship or had a loved one die would be seeking help.