Can i tell ye one thing and not boasting

It's having a loss of purpose and or feeling so bad and down that you have no energy. It's losing your optimism. The tiredness affects you.

Resilient people have 3 things in common. They are hopeful about the future, good coping skills, and not taking all the blame for the hard times. Obviously, you must have good coping skills, have some enjoyment in every day, and you are hopeful about the future.
Depression has 3 causes negative self-cognition, childhood trauma, and can be familial or genetic.
Some symptoms are overwhelming sadness, hopelessness, lack of joy in activities that are usually enjoyable, significant weight gain or loss, sleeping problems, and irritability. (Possible suicidal ideation)

How i used to feel depression, as a veil that would keep me under the surface of life. Like, I couldn't truly laugh, couldn't truly hold onto anything uplifting for long, i needed a ton of alone time as I felt i couldn't relate or keep up with others ..

you may have done part of your program. now you need to pause and change your worldview.

go for a walk in the woods or ride a bicycle. your brain will clear and new thoughts will emerge. in time.

Both previous posters hit the mark.

That’s an interesting question but a hard one to answer without it referring back to itself

Lack of enjoyment in life
Negative thoughts
Helpless thoughts
Lack of interest in the usual activities of life
Isolating

I think we all get depression... depressed... at times. Some people get more depressed and for longer periods of time. Some people pick themselves up and carry on while others seem to fall further into sadness and negative feelings.