That is a good question and one that can take time and effort to resolve.
In terms of herbal support you could try taking Passiflora complex which is traditionally used for reducing anxiety and nervous tension. It generally takes from a few days to a week to feel the benefits.
If you haven't noticed any improvement after a month we would suggest you try an alternative and perhaps seek support from your GP or healthcare provider if you have not done so already.
You could also try taking a magnesium supplement in the evening as magnesium can calm the nervous system, promoting relaxation.
If it is low mood that is the main symptom you could try Hyperiforce® tablets (St John's Wort) which are used to treat symptoms of low mood and mild anxiety. You need to allow 4-6 weeks to feel the benefit.
St John's Wort is contraindicated with a variety of medication so it is important to refer to the patient information leaflet and your GP if you are unsure whether or not it would suit you.