I’d say it’s a mixture then of just the environmental factors (Re: Misery loves company) and codependency. Like you said you couldn’t tell them out of concern that their mental states would go for the worst if you did. You’re willfully denying yourself the freedom of helping yourself for the sake of others, and that is at it’s core, being codependent. A little bit of it is good, but with regards to how you’re taking it, it’s dangerous.
That’s too much pressure for one person to take. Especially considering you’re 18 (I’m guessing the youngest out of the family?), it’s not exactly fair that they lean on you at a young age for emotional stability, and on top of that, they badger you when you’re down? Yeah, no wonder you’re depressed.
So I would lead to believe that that would be the exact root cause of how you’re feeling. If you’re spending a lot of time at home, then getting out of the house is probably a good counter for the stress at home.
Getting a job is one way to get out, but bear in mind that if the job stresses you, it may make things worse instead of better. Plus, you still have to come home sometime. But on the other hand, you can make friends through working, so that could benefit you in more ways than just getting you out of the house and making money. Just monitor yourself, you know how you’re feeling better than anyone. If the job starts making you feel worse, ask if you can take on less hours. I wouldn’t quit outright, because that would just stick you back at home with all this going on. If it’s the job itself, then look for another place to work (which I would advise you to do before you quit the first one).
But I do know one thing, staying in that house all day would be the worst thing you could do for yourself.
While you are severely depressed like this, I would also advise you to take medication, but if you start feeling like the situation is going better for you, I would consider weaning off of it to see if you can handle this without the medicine. If you feel yourself slipping back into depression, get back on it. A counselor would also help you decide when it might be time to wean off.
Hope that helps in any way.