So it is my personal story when I was in my teenage I used to be a guy like I was told to be by so-called elders, teachers, etc. that we should be kind to others and not make fun of others it is bad, BUT I/we was never taught that when someone trash talks to you you have to answer back in the same tone. We were always taught to be kind. And top of that as boys we have to hide all our emotions be it of any kind and irrespective of any mental stress we are going through we have to keep a smile on our precious faces that is not any more precious, you can't show your feeling out in open. And one more thing when you feel for someone you can't even tell her because that you have to maintain that rough personality to impress that society that doesn't even know that you exist. 

Till when you realize that you are falling into something and you can't come out of it done it's late. being a teenager it takes only one part of the moment to push you over the edge and hence by the time passes that little seed of self-doubt starts growing and by the time you realize that you are changing or to be precise your thought process is changing the self-doubt has grown soo much inside you that it will take too long to clear from your head. It is not like that we don't want to but something or the other keeps pushing us back to that box where there is only self-doubt and nothing else but fear of ridiculous things which is important at that time. 

Once when I was in college I liked a girl very much it was hard to confess to her because she was like that girl that every boy fascinates about being her friend or boyfriend (if your luck is damm good) so before taking that huge step I asked my friends about what I was going to do, they said: “just do it ."

Every day I used to see her gaze over her, which I think at a certain level she knew I like her but can't take risks, so I continued to do what my friends were saying to me and when I did what was told by my friends to do it and rather than saying no she pointed out things that I already knew I was not good at (pointing out my insecurities) and mocking me like anything, and that didn't stop there next day her friends started doing the same mocking, it continued for 2 days but those were like growing of seed of self-doubt and it continued but as time passed the self-doubt also started to decrease.

If you are also having self-doubt you can also come over it by just one simple thing that is doing those things in which you are good at and sure that you won't screw it, keep doing those things, and boom you are new as confident as never.