Take a week off.
I swear, take a week off, right now, fuck anyone else whose priorities don't include your well-being. You sound like you're in a pretty dark spot and there's nothing you're going to get done at work in the next week that's more important than sorting out what's up.
Are you near family? Are you near friends who are good enough to be family, but don't work at this job? Get one of them on the phone, tell them what's up, that you need an afternoon of their time to relate your troubles. And unburden yourself.
Hopefully they can give you some good insight, because they know you so well, but it doesn't really matter what they say. This is rubber duck debugging for your life and well-being. You're exhausted and it's dripping through your post. Taking some time to work through what has you so miserable will be crucial to you getting some peace.
If you can afford it, fly or drive somewhere for a couple of days. Turn off your work email if it's delivered to your phone. Get some perspective. It's easy to believe, especially in startup land, that your job is your life. It isn't. It isn't even remotely close. One day when you upload your brain to a digital substrate and instantiate your consciousness in a virtualized environment, you won't even bother remembering this.
None of us can tell you what you should do about your career or your job at the company. But I can tell you, emphatically, that you need some distance from the day-to-day of this job before you can make good decisions for your life. That you care about your friends is admirable, and I respect your passion for getting them out, too. Whoever hires you with the ability to truly see your value is going to be especially lucky. But right now, you can't worry about them. Focus on what you need. Once you're okay, you can work on them.
Others have suggested you see a doctor – I would agree with this advice as well if you feel you're in a downright dangerous spot with regard to your mental health.
edit: Related only through mental state... (cool story, bro alert!)
Around this time last year, my "fuck working for the man, I'm going it alone!" plan had gone pear shaped because I suck at networking and chose a cheap but isolated town as my new home. The plan mostly backfired because it was predicated on my finding some part-time work after moving. Only problem was: 20% unemployment in my new town. Oops.
So, despite making some decent money from iPhone app sales, I wasn't growing my revenues fast enough because of this unexpected hole in my income. Burned through cash that should have lasted a year in about six months.
Miraculously, after six months striking out despite solid credentials and work experience, a place finally got back to me about a part-time position.
At Target.
I love customer service, so the cash register bit was extremely fun for me. Stocking shelves and tidying the store, though, that made me miserable. Tedious and Sisyphean, since it was the holidays and kids would destroy the toy merchandising you'd just spent half an hour on.
I told my management I hated the merch work – and they respected my passion for doing customer service. But I'd be farming the store for another two weeks. That's how the schedule worked.
A week of eight hour shifts like this brought me to the brink. I was miserable. When I wasn't at work, I was constantly dreading my next shift. It was a distraction from my development projects, which ceased to give me joy. I was unkind to the people who care about me. Perhaps immaturely, I was especially ashamed to be making minimum wage for the first time in my life – even in high school I'd done better than this.
So one night my shift starts. I'm an hour in. Yet more merchandising misery. I can't fathom another six hours of this. I'm at my breaking point.
So I smile. I realize that, while I often forget this, I am free.
I push my tub of orphaned merchandise out of anyone's way. I walk to the store's office area and check in my radio, keys and scanner. I find the general manager and explain that I have to leave immediately and I'm not going to come back. She was surprised and confused – she'd complimented my work several times. She explained if I left like this, Target wouldn't ever hire me again.
I was okay with that.
I signed some papers and left. On the one hand – I couldn't believe I was doing this. I'd always been Mr. Responsible. Always gave ample notice. Always the guy you could count on. But in the end I knew that my well-being was more important than being the guy for some retail store in the middle of Oregon.
My troubles, of course, were just beginning. I still needed money. I found gigs teaching people Excel, fixing people's Macs, but eventually the cash all ran out completely. But by then I'd found work in the Bay Area, I relocated, and all was well.
</cool_story_bro>
So in the end, if you're truly miserable, re-assert your control over your own life. It sounds that, like me, you've lost track of the fact that you're running the show. I can only speak for myself but the jolt of energy I got from doing what was good for me was incredible.