As others have said - this isn't being censorious or moralistic about drugs - but step 1 is stop smoking the pot. Your mind WILL clear up after a week or two being off it and after a month or so you'll really notice the difference.
Contrary to movies like The Social Network, getting drunk and stoned all the time doesn't fit with the stresses of running a startup I'm afraid.
Burnout is an under-talked-about problem with startups. You are far from unemployable coming out of a startup. You just need a break and some time off to get your head clear. Depressive tendencies are not unusual for the kind of person drawn to entrepreneurialism.
If you can then take a break from your startup. Tell your co-founder that you need a couple of weeks off or you're gonna burn out. If they can't make that work then you don't want to be in a startup with them anyway, as there'll be more ups and downs and you need to have each others' backs during the tough times.
If this startup isn't working for you and you have no other option, getting out is not as drastic as it seems. The company will survive. You will survive. Things will not be as dire as you think. It just feels that way now and the combination of pressure and anxiety makes it hard to get a better perspective.
Being stuck in something because you feel financially bound to it is not a good thing - you'd be surprised the options that are available to you but it's hard to see them when you're in the frame of mind you are. That's not critical :-) ...it's a good thing - it means that when your mind starts to clear up you'll see many more options.
I've been in Silicon Valley for about 14 years now (really? damn I'm getting old). I've seen, and been through, every up and every down. If you want someone to talk to for a bit of perspective, I'm happy to listen any time. (Add a d in the middle of the two names my username at gmail for my email address). Perspective can help :-)
Either way, don't get too down. It's not as bad as it feels right now I promise.