For whatever reason. E.g.: - your job sucks - your relationship sucks - sucks that you don't have a relationship - don't make enough money - unemployed - nobody around you gets you - etc...
What do you do?
You're under it and it seems like nothing you can do - which is essentially true.
That is - it's hard to fix "bad weather" to change it to "good weather".
The universal truth about bad weather that eventually it is going to change to good weather.
Although knowing this doesn't help when it's pouring rain, cold and miserable all over.
The action plan - is the fact of bad weather presence does not mean that you need to stop living.
And it also does not mean that you need to stop doing stuff you were doing before.
And it also mean that you don't really need to change the weather but keep doing your best to keep going your way.
In other words - it sucks, but let it be and keep going.
The worst thing to do when weather is freezing - is to sit and wait for weather to change. We all know why.
So, let it be.
Keep going.
That's it.
With the latter, you can work out a plan of action to gradually lift yourself out of misery. Since you're on HN, you're in a community of like-minded perceptive, intelligent individuals. There's an abundance of advice in previous threads on this subject.
The former is more complicated, and potentially dangerous. It's classified as a 'mental illness'. Professional assistance may be called for; other times one can recognise the effects, realizing that the cause is within rather than without. If a good night's sleep doesn't cure it, seek assistance...
When I was in Berlin, on the rare bad day, I would go to my closest train station, sit on a seat and watch the people go by. Inevitably someone would come up and ask me for directions. Helping someone even with my terrible German skills cheered me up.
These tricks aren't long term fixes (walking & sugary mochas won't help you earn more money), but they might give enough of a short term boost that you can get back to working on something productive, and thinking about how to fix the bigger issues (improving relationships, finding a better job, making new friends etc).
Just look at all the gifted child posts and their comments, it's super sad.
I don't get it.
Also, pick up local news paper and take a look. You will find enough suffering and pain in them to make your life look like a walk in the park, or in short, philosophy.
A sense of depression (and I assume you're referring to the emotion, not the medical condition) quite often has nothing to do with easily identified circumstances, the mind just tends toward the things it is dissatisfied with.
If you believe you have the medical condition, treat it like anything else: see a doctor. I know many people who have wasted years of their lives fearing a few moments of awkward discussion with their general practitioner. There's no rational reason to join them.
At this point, I'm just passing time with cigarettes and alcohol telling myself it's just Seasonal Affective Disorder and winter will be over soon.
I know this isn't very helpful, but maybe you can find solace in knowing that you are not alone.
For the specific questions, solve them. You are not married to your job, your boss, your girlfriend or your city.
You wouldn't believe how helping can be splitting wood with axe.
Then I shitpost.
Sometimes I game.
It's amazing to see that despite the amount of reading I do to become smart, I'm dumb as a brick. Maybe I should do some reading about that..
I look in retrospect at all the hard times I have been through, recall the state I was in and remember that at that time I invariably said to myself that in a few days/weeks/months all of that will fade away. I have been right so far. Life goes on and whenever I'm in a tough situation, I know that it will either get worse and I should enjoy it before it does, or it will get better. It also helps to keep in mind the following: When all of this is over, would I have reacted like the ideal I'm striving to be would, or in a way beneath that ideal? I also live while a 100% conscious at all times that I could die right now. That's why death doesn't shock me. That's why I have always wondered how people are somehow "reminded" it exists only when someone dies.
That's also why when I party, I do so like I'm both ephemeral and eternal. And why I hug the loved ones. That's why I look at the sea like it's the first time, every time. It's why I'm happy to wake up; because I'm not dead yet and when you die there probably aren't good books and showers. Though it's also why I'm sad, because I could die without yet having achieved anything of importance having wasted so much organic matter and emotional investment from others, and no contribution in the slightest to entropy to even deserve to die. But then again, all the more reason to live another day and try to do just that.
Some people also go for entertainment. Here's a quote from Blaise Pascal's "Pensées" (it's an amazing read. A brilliant mind.) where he addresses entertainment:
>Whence comes it that this man, who lost his only son a few months ago, or who this morning was in such trouble through being distressed by lawsuits and quarrels, now no longer thinks of them? Do not wonder; he is quite taken up in looking out for the boar which his dogs have been hunting so hotly for the last six hours. He requires nothing more. However full of sadness a man may be, he is happy for the time, if you can prevail upon him to enter into some amusement; and however happy a man may be, he will soon be discontented and wretched, if he be not diverted and occupied by some passion or pursuit which prevents weariness from overcoming him. Without amusement there is no joy; with amusement there is no sadness. And this also constitutes the happiness of persons in high position, that they have a number of people to amuse them, and have the power to keep themselves in this state.
>Consider this. What is it to be superintendent, chancellor, first president, but to be in a condition wherein from early morning a large number of people come from all quarters to see them, so as not to leave them an hour in the day in which they can think of themselves? And when they are in disgrace and sent back to their country houses, where they lack neither wealth nor servants to help them on occasion, they do not fail to be wretched and desolate, because no one prevents them from thinking of themselves.
PS: When I was 17, I became aware of what Gauss did when he was my age. I've been "depressed" and doing that ever since. Now cheer up, you could be me!