All I can say is you are not alone. When I read this, almost all you say resonates with me - and there is no privacy, especially in startups. I learnt it the hard way too.

Please don't think you are not good enough. The interviewing process is just ridiculous and illogical (for an industry that deals with logic all the time!).

I don't have any answers - since I am also in the current state. And I also found out that no one has any answers - everyone is probably lost in some way or another.

Take some time off if you can. And if you have the luxury, don't think about anything else for a while.