Depression
TESS
It is important to distinguish being sad and grieving from suffering from a depressive disease. Everyone has felt sad at some point. Your existence then seems sad and insurmountable and the future looks hopeless. It's natural to feel sad and melancholic when you experience adversity and loss such as in the case of an unhappy love affair, unemployment, or bereavement. When you are "naturally" sad, you don't come to a complete stop. You are able to take part in activities even though "your heart isn't in it". And when something good happens, you can still feel happy about it.
It is not abnormal to be naturally sad and it is not a disease. Sadness and bad moods will pass. Even if you are experiencing serious grief, its severity will lessen with time but perhaps your grief will never completely go away. You can do something to help yourself and support from your family and friends will also help you. You need to be with others and speak with others whom you can trust about your problems in order to get better.