Depends. Depending on how bad your mood is there are a number of options. The best studied for mild depression is st johns wort but if your mood is only sometimes bad then something milder like chamomile tea can help. If you are having insomnia then melatonin might help you sleep and better sleep will lead to better mood. Exercise is also an excellent antidepressant. If more severe, then same can help.
Some herbs can hurt. Do not assume that all herbs are safe. I have treated many individuals who have developed various psychiatric symptoms including psychosis after taking herbs, excessive amount of supliments or energy drinks. Some herbs has significant interactions with medications. It is important to talk to your doctors about herb use.
That's 3 problems. A great deal of effort has been expended to find herbal remedies for all three of those issues; mood instability, depression and nervousness. So far there are no outstanding remedies that are strongly recommended. St. John's wort for depression, chamomile tea for insomnia and anxiety , sam-e for depression have all had their day. They are only weakly effective. Be careful they can be harmful also.
Aromatherapy. Aromatherapy can also lift the mood and calm the nerves. The amygdala, a part of the limbic system in the brain, can be stimulated by the sense of smell. Great reference: "Modern Essentials: A Contemporary Guide to the Therapeutic Use of Essential Oils." Essential Oils can be therapeutic but it is important to consult your doctor/ pharmacist to make sure there are no medication interactions.