If you're suffering from depression, the book 'Feeling Good' by MD David Burns can help.
It's about cognitive-behavioral therapy, a successful treatment for depression, dealing with negative thinking patterns.
Link to PDF: http://www.mediafire.com/file/sdydy3n2mdpueck/%2528harper._health%2529_burns_david_d_-feeling_good__the_new_mood_therapy_revised_and_updated-avon_books_harper_%25282009%2529.pdf/file
(if you don't want to use Mediafire, look for it on libgen.is
- it's a good site for PDFs in general - browse/download at your own discretion).
Also listen to https://www.youtube.com/watch?v=qmDwl2-IZEk
It's an audiobook of 'Feel the Fear and Do It Anyway' by Susan Jeffers, a self-help book on overcoming anxiety.
You can also try some St. John's Wort (it's a plant) tea for immediate relief (I mix mine with Melissa for the calming effect).
It might also be nice to learn to meditate, as it can relieve your mind of negative thoughts. nperov.com
for learning materials.
Or maybe you just have a vitamin D defficiency, see https://warosu.org/sci/thread/S10221251
Go get some sun.