Despair in a dream usually reflects a sense of being stuck -- exhaustion that's gone on long enough that the direction forward has become truly hard to see. Unlike ordinary sadness, despair in a dream often names a specific loss of momentum, a feeling that effort itself has stopped producing any visible change.
Entrust the Sun when you are depressed because it means renovation, growth, and quick response. - Picatrix (Andalusia, ~1000.AD)