First off, your thoughts of SI show something isn't right, so please see a doctor and tell them what's going on with you.

Regarding your question, depression varies, as already said. For me, as bipolar, it's as if I've suddenly been placed into a vat of slow setting concrete that no one else can see. Everything I do requires a massive amount of physical effort. I go to sleep and don't wake up. I drop about a hundred IQ points. I begin to put weight on fast, in excess of fourteen pounds a month.

Once that's underway, then my mood follows, and all the things that come with it (depressive thinking, self-isolation, thoughts of suicide).

But... I don't SI, so you see how it's subtly different for everyone. Get help and advice now, not later. Give yourself every chance to recover.

Hope that helps