This is something I wonder about simply because I find that people who appear exceptionally confident often turn out to be masking deep insecurities. And ESTPs do appear exceptionally confident.
I'm certainly not suggesting that all or even many ESTPs are deeply insecure - and obviously anyone of any type has their insecurities. But if the ESTPs or those who have been close to them would like to weigh in - I'd be curious as to how and why ESTPs are insecure, where you do have those vulnerabilities and fears.
Would it perhaps be in the area of feeling that you have a hard time understanding others' emotions, so you're afraid that your interpersonal skills might let you down unexpectedly? Something like that? Or am I way off?
Please do share if you feel like it.