How can a startup founder remain mentally healthy?
Founder's emotional state often reflects the state of their company. When your identity growth with your company and you encounter things that are completely outside of your control, those things affect you emotionally, making you irrational, and an irrational founder is not useful. It is healthy for you as a founder to disassociate yourself from the performance of your company, and from how the world treats it.
Recommended reading: Keep your identity small. by Paul Graham.