What to do when you feel you are burnout

Hi Ben, I know there are two podcast episodes that talk about preventing burnout. But unfortunately, I think I'm already there. I thought I could manage it, but things outside my control happened. I'm a startup co-founder and we are in a crucial point at the moment, where I need to be present, but my brain is exhausted. I have constant headaches and I reach cognitive fatigue very quickly, so I can't think properly or perform the tasks I need to do. I can't be productive and it's hard to relax. Do you have any insight of how to start treating a burnout?