Feeling down refers to feeling sad or even depressed.
you are feeling good about yourself
1. Happy feeling 2. Sad feeling 3. Nervous feeling
Even when I'm feeling down and it seems you are far away...
The flu left me feeling dizzy. Feeling dizzy, I sat down.
They are feeling the effect of friction.
"Feeling blue" IS an idiom - you cannot literally feel the color blue! Feeling blue means that you're sad.
The idiom "feeling down in the mouth" means feeling sad or unhappy. It refers to someone looking sorrowful or melancholic, with their mouth turned downwards indicating their emotional state.
if your sitting down
The word you might be looking for is "dejected," which means feeling sad or disheartened due to being put down or discouraged.
The metaphor "down in the dumps" means feeling sad, gloomy, or depressed. It is often used to describe someone who is feeling low or unhappy.
Kenwyn House & Louse Down & Tiffany Page - Love Is A Feeling