Task: Assess the emotional content of the tweet and classify it as either 'neutral or no emotion' or as one or more of the given emotions (anger, anticipation, disgust, fear, joy, love, optimism, pessimism, sadness, surprise, trust) that best represent the tweeter's mental state.
Tweet: When you burst out crying alone and u realize that no one truly knows how unhappy you really are because you don't want anyone to know
This tweet contains emotions: 
sadness