There's a particular kind of isolation that founders experience that's different from ordinary loneliness. You're surrounded by people who depend on you, and precisely because they depend on you, none of them can be the person you're fully honest with about what it costs to hold everything together.
So you perform. Certainty in board meetings, calm in all-hands, optimism for your co-founder. Over time the gap between what you project and what you actually feel becomes its own source of exhaustion, sometimes the biggest one.
This isn't something coaching or "mental fitness" apps are designed to address. It's a structural psychological burden that comes with the role, and it usually needs proper therapeutic work.