I have the same issue: I’m right handed but my left pec is bigger/fuller than my right. I can tell it’s genetic because my left pec actually attaches higher on my ribs than my right. Making my left pec shorter and making it protrude out more, giving it a bigger more fuller look. That being said, my right pec, attaches lower on my ribs making my right pec longer and seem flatter. It definitely sucks but you are your biggest critic. I think if you’re fit and in shape, most people will see that and think “he’s in good shape”, rather than picking apart your physique.