I'm burning out and I can't tell anyone

I'm the cofounder of a startup that really needs me right now. I suspect I may be burning out or have already burnt out. Despite things going relatively well, I'm somewhat depressed all the time, easily irritable, resentful of my cofounder and constantly think to myself that I need a break.

My work isn't really getting affected (yet?). I've taken half days off before, but they weren't enough.

Anyone had the same problem? What did you do about it and what happened afterwards? Thanks.