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.
The Sun answers those who are ruled by Mars, and Mars answers those ruled by the Sun. - Picatrix (Andalusia, ~1000.AD)