Of course, we know that positive emotions are good and they create positive outcomes and actions, we see this everyday of our lives. Negative emotions, by their nature however, act in the opposite way, causing negative physical outcomes and effects in our lives. For example, anger can cause violence and sadness can cause despair, lethargy and procrastination.
But that is not to say that Negative emotions, do not have a meaningful purpose in our lives. In fact, just like our nerves react with pain to alert us that we are hurt, thus protecting us from further hurt (for example, if you cut yourself, you will feel the pain instantly and quickly move to wash and apply a dressing on the cut, whilst also quickly moving away from the object that caused the cut in the first place), then similarly, negative emotions arise to protect us and alert us to a negative situation, so that we can then quickly do something to resolve it, thus restoring us to a happy or contented state of mind.
The difference with emotions however, is that they are caused by a neuro-chemical or neuro-electrical reaction within the synapses of our brains. Therefore, once we are alerted by the emotion that we need to do something about a particular situation in our lives and we start to do what is needed to resolve it, the emotion itself still stays with us because the neuro-chemical reaction takes much longer to subside. Therefore, we may feel sad or angry for much longer about something that has already been resolved. So we no longer need the emotion - yet it is still there. Recognising this idiosyncrasy of emotions is the first step to learning how to deal with them in a practical and logical way so that rather than causing long term harm to you, you can enable them to serve their purpose of alerting you and then safely contain them within your psyche.
Whilst the onset of an emotion may be automatic, as a reaction to an event in your life, the control of the emotion once it is 'awakened' is in your hands. However, you need inner strength and conviction to take that control and use your emotions in the way that you desire.
This video describes in detail, this process and what you can do to restore your happiness, your faith in yourself and your level of contentment to its rightful state.