Unfortunately, people just say stuff (teachers, bullies, others) and you only realise years later that some of them say it for kicks, because they are nasty or just misguided and have no idea. It takes on a huge significance in your mind for years. It is all completely meaningless, really, but impossible to get out of your head.
Google imposter syndrome.
Go and see the doctor.
Out of interest, why do you mention about 26 weeks? I've met a number of people who were born prematurely (I was) and most of them read very deep significance into this fact and it's an essential part of their identity, often being seen as 'sensitive' by their parents and mollycoddled and not allowed to do things and seem to me to be examples of a self fulfilling prophecy. A lot of stuff parents can say on being born prematurely is unfortunately the utmost nonsense.