After my depressive episode it became so clear to me, that for some conditions it is so important to publish both external and subjective symptoms! Sometimes how things manifest on the outside is completely different how they feel on the inside! All the symptoms of depression I see in the internet are about how it looks on the outside. This makes it almost impossible to self diagnose until things get rather extreme (not being able to get out of bed, etc).
These are my subjective take on how the external symptoms may feel to the sufferer (as long as what I suffered from was indeed depression and not psychosis):
Social isolation
How it feels: you start getting disappointed with some people you used to have around you. You feel that some people started disliking you for some reason, and some other just feel like are about to let you down anyway, so you would rather limit your interactions with them not to be hurt. And with the people that still matter a lot to you, however, whenever you are around them you feel that you are somehow dragging them down, that you are a burden to them. You feel you cannot be there for them fully, so you decide to give them a break from yourself for a while. Or, it may feel that everyone just stopped being interested in how you are and is not reaching out to you anymore. Or, you may just feel like you are about to get sick just when you were about to meet someone. Or just remembered that important thing you needed to do. Or a mix of all of those. I guess the reasons will be different for everyone, the point is: your brain is finding reasons for not seeing anyone, even if you do not feel like isolating yourself at all.
Having no energy
How it feels: you realise that you have been doing too much recently and you really need a rest. You simply need to take care of yourself. And oh boy, this resting feels like ages, you must really have been putting too much on yourself, so now you need to "rest it all" out. You start cancelling plans, in order to have more time for rest, as clearly, you must have overbooked yourself.
Feeling down
How it feels: it really strikes you how shitty you just did your job. And how bad you are at what you are doing. And how unlikeable you are. It does feel unpleasant, but you decide to take it head on, and accept that critical feedback your brain just gave to you. You start looking at your other actions and realise that you cannot actually do almost anything right and fail at everything. And before you can gather enough energy and will power to try to improve it, another hit comes. You start feeling guilty and ashamed most of the time, and feel that it is completely justified, and cannot understand how come you used to feel so optimistic about yourself in the past.
Feeling suicidal
How it feels: having ruminated about how shitty you are and how your friends feel betrayed by you not reaching out to them anymore (which you completely don't get since nothing changed in how important they are for you) you start getting the idea that it would actually be better for everyone if you were dead. The feedback your brain is giving you has changed from telling you how shitty you are to how pointless your whole existence is. You start imagining how it would be to die. Not that you want to do it, you do not! But you are just curious about the topic. And it is actually quite interesting how it feels to die by hanging yourself, isn't it? And what if you discover that it is not as scary as you thought? One day you just "feel" it in your whole body, that this is what you "have to" do. It feels like a feeling of purpose, a mission. You do not know why you are doing it, but this is the only thought that makes you feel so energetic like you have not felt for a long time. If it does feel so, it must be the right things to do, right? ... and maybe at the very last point, you look back on your life and remember the people that mattered the most to you. You start crying feeling that wrenching hole in your heart already, the feeling of having lost them. And you wonder: would they feel the same? Is this connection with them real, or is what I'm doing real? Which one is real? And maybe then you remember someone's honest "I care", one that really went to your heart. Or maybe you'll remember someone's "can't you just get yourself together?!" instead.
That are just a few.. actually all of them are about a social isolation in a way. So now my personal depression gauge is: how many times I met with a friend over the last week? This is where the spiral starts.
Depression is like a computer virus getting into your brain's matrix software. What you think is happening is not what is happening. You cannot trust anything but one thing: that its program is to bring you to a self destruction, in one way or another.