So it seems quite a few people who are reasonably sure of their dominant function are not so sure of their secondary one. This is interesting, because the effect on behaviour might be different to that experienced by someone whose 'doubt' lies in the E/I area, therefore not really knowing what your actual dominant function is. If it's supposed to be dominant, you'd think we wouldn't miss it! So how come we do?
I've often heard ENxP's say similar to what Vortex said there, myself included - that they were quite withdrawn and tended towards 'sedentary' hobbies as a child, such as drawing, reading, writing. And later in life developed a new sort of confidence, the lack of which seemed to be all that was holding them back before from fully engaging in the external world with a thirst for exploration both physical and social.