It took a lot of development and experiences to help me finally realise how to accept that I have a fairly decent-sized penis without a) feeling like I'm bragging, or b) deluding myself. I am in no way huge, but through a strange combination of personality characteristics and life experiences, I thought I was average-sized or even on the smaller side throughout my early sex life, let's say from high school up to about my mid-20s. When I received comments from sex partners about my cock being big, I thought they were just being nice, or maybe they had less experience. Even in the face of cold "hard" facts, such as regular condoms squeezing me so tightly as to make sex unpleasant, I still thought that's just they way it's supposed to be.
There was no single awakening moment that got me there, but along the way I had some pretty big clues, like the first time I measured myself fully erect in my early 20s, or the first time a girlfriend teasingly bragged to her friend that "he rocks my world in bed, and he, you know..." and then gestured about my girth. And the first time I used a larger-sized condom, which was still uncomfortably tight but much better that the other condoms I'd used up to that point, I guess I was about 26 or so when that happened. I do wish I'd become aware at a younger age, but everything happens for a reason!