I have not doubted my dx at all for at least 15 years. In the early couple years of the 15, though not doubt, I did believe I could fight the disorder on my own (without meds), to varying degrees. Soon after, I realized that was not possible anymore.

Prior to my official diagnosis, I knew something was wrong with me, at times (during depressions and mixed states, notably), but when these periods eased/ended, I looked back at them as simply resolved reactions to hard times/stress or "brain flues". But these experiences would always return. New strain of "brain flu" or life challenge? Nope. It was a permanent mental health condition, characterized by remissions and recurrences.

In my early years (ages ~14 years old to 31), I had little to no insight into my manias, and especially hypomanias. It's astounding how that is. Yes, ramifications resulted, but ohhhhhh...How the brain can make excuses and pass blame! When grandiosity is included, it's easy to look at some behaviors as justified, or a sign of superiority/greatness/adventurousness instead of shocking actions. To me, when at perceived best, THAT was my prefered baseline. Anything below or painful, was something to fight or improve. Others in my family showed similar behavior. From within my family circle, some bipolar behavior became normalized or labeled "personality trait". Depressions minimized or just temporary angst. Or, I was often literally away from those who could compare. My later severe episodes were so horrible or shocking that the illness was undeniable. Ramifications were beyond sweeping under a rug.

Obviously, the most extreme episodes are good convincers. I have bipolar type 1 with 11 hospitalizations in my past, plus a series of ECT, and MAJOR incidents I could describe. You do not want to have to get to that to be convinced. Believe me!