One reason why people find it difficult to stay in the present moment is that they don’t understand that their thinking changes in a low mood. Never believe your own thoughts when you are in a low mood otherwise you will frighten yourself with your own negative thinking/imagination. Watch how when the low mood passes your thinking becomes better. It's important to realise that sometimes thoughts may not be real or factual, if they can change with your mood. They may just be a perspective that changes over time. This interesting quote from
Shakespeare “There’s nothing good or bad but thinking makes it so” reflects this concept from Mindfulness, that thoughts are not facts.