I have had depression for ever. When it was really bad, it was like looking down a tunnel, and there was a 'smell' in my mind, that told me.
I have taken all sorts of pills, none of which seemed to work, but were all the doctor had to offer. Nowadays you can get CBT, which lots of people say is very useful.
About 20 years ago, I got prescribed fluoroxetine, and the change was amazing. This works when the depression is caused by a lack of a hormone serotonin to the brain. In other words, a physical thing.
I can still get bad days, like anyone, but I can usually cope till they pass.
Don't know if this is any use to you, but it is what happened to me.