You can tell your co-founder. You've got a duty to let them know. I'd be furious if a co-worker told me that they'd been nursing a huge problem on their own, and not let me in on it until it was too late, and it put the whole project in danger.
I'd time-box it - tell them you are feeling burnt out, and want their help for 2 months. No reasonable person would be annoyed at a co-worker taking it easy for 8 weeks. If you feel better at the end of it, then the crisis (for them) is averted. If not, you can consider your options. Note - I'm not a therapist, and I don't know if that's the best way of handling it. There's plenty of people who are better qualified than me to come up with a good plan for this.
> startup that really needs me right now
You should really re-consider that. No startup needs 100% of your ability. A slightly inferior feature-set won't kill a viable startup. You aren't in a 100m sprint in which you need to do everything that's humanly possible to beat the other guy. It's more like a dance competition, in which strategic decisions are more important. That's assuming you have a ton of competitors.
Your boss is a dick, so he's making it clear they shouldn't need you. Get that shit out of your head that the sky will fall for these people if you leave. It won't.
It's hard to believe, I know, but they WILL find a replacement and life will go on.
If I'm wrong, your boss should've learned not to work you like a whore.
I'd time-box it - tell them you are feeling burnt out, and want their help for 2 months. No reasonable person would be annoyed at a co-worker taking it easy for 8 weeks. If you feel better at the end of it, then the crisis (for them) is averted. If not, you can consider your options. Note - I'm not a therapist, and I don't know if that's the best way of handling it. There's plenty of people who are better qualified than me to come up with a good plan for this.
> startup that really needs me right now
You should really re-consider that. No startup needs 100% of your ability. A slightly inferior feature-set won't kill a viable startup. You aren't in a 100m sprint in which you need to do everything that's humanly possible to beat the other guy. It's more like a dance competition, in which strategic decisions are more important. That's assuming you have a ton of competitors.