I do not know for sure how much I actually know about the subject, but from the things I've been through myself/am going through right now/read online or in books/talked about with multiple friends who have been diagnosed with depression, I see it this way:
A lot of times people tell others that there is a light at the end of the tunnel, right? When you have depression, the actual illness, not the 'mood' so many teenagers tend to have when they have a lot of homework, you don't see that light. The tunnel just looks like an endless, dark road without end.
Of course, there are multiple versions of depression and sometimes it can bring other things - like insomnia, for example - with itself as well.
It also depends on your character's brain. As you might know, depression doesn't need a reason. It's a chemical unbalance in your brain. Many people are unaware of this (hence why a certain amount of people are always surprised when famous artists are dealing with, or even pass away from depression/suicide. They have a perfect life, right? But that doesn't change anything in their brains, it might even make it worse)
But insomnia, for example, tends to occur more for people with a high IQ, for they are unable to shut off their brain. They are always thinking, their mind is constantly working, which robs them of their sleep.
Lastly (sorry this is so long idek if I'm making sense), people who suffer from depression often get the feeling that they are not worth it. They start caring less about taking care of themselves because they feel like they are not worth it. They apologise a lot, are afraid that they annoy people, etc, etc.
Depression is an exhausting illness. Especially if your character has a daily job or goes to school, they have to fight their own mind every single day. Getting up from bed can be insanely hard, for they would rather stay asleep, and sometimes they wish to not even wake up anymore. They lose interest in things they used to love and feel like they have nothing in life to look forward to. Almost nothing brings them joy. Of course, sometimes there are good days, but depression is quite a nasty illness that doesn't randomly leave your brain (sadly)
Okay, this is really the last thing (sorry lelz):
A lot of times, people who struggle with depression call for attention. Not in a bad way, but in a way that's more a cry for help than anything else. They might let off that they're feeling bad, in real life or they post it on social media. They might make small, seemingly harmless comments to their friends a lot, in the hopes that somebody will notice: ''I haven't eaten today'' ''I barely slept last night'' ''I'm so tired''. They often hope that someone will notice and will be there for them.
I hope this was of help, sorry if it didn't make a lot of sense. My own head is a bit full of a lot of things at the moment, so I keep losing my train of thoughts mid-sentence lmaoo
^.^
(this is of course from my point of view in life, so if I'm horribly wrong about something, please do tell me and I sincerely apologise in advance if that's the case…
…stay strong everyone, you're all amazing, we can all get through the bad shizzles! c:)