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.