Everybody is insecure about something. Everybody has at least one or two things that they're nervous about and think about. It's hard to walk into school or work or life every day and be 100% confident in yourself, how you look, what you have or who you are as a person. It's insanely difficult to feel comfortable in your own skin when the world around you is telling you to be you, but to be smaller or bigger or taller or shorter or louder or quieter or whatever society tells you that you're not enough of.
The biggest thing to always remember is that no one, even the most confident person in your entire world, is confident all of the time. They have something that they're unhappy about or that they aren't sure about. Everybody does. That is the first thing to think of when someone makes you feel insecure or bad about you - they have something that they are masking up with that. Pity them. Actually, feel bad for them because it's sad that they have to tear you down to lift themselves up. That's never how it should be. Someone who's confident, or trying their best to be confident, will lift others up, support others, try to build confidence in those around them, especially those who look nervous or insecure.
Working on your confidence is hard. It can take so much out of you to even put on a smile and walk into a room - whether it's a classroom or an office or wherever - and feel somewhat okay with yourself. But you need to take steps to do it. You need to realize that you have things that other people don't.
News flash: you're you. And no one else in the entire world has that. Confidence doesn't happen overnight. Sometimes, it never fully happens at all, but what you can always do is work on it and find it within yourself to have enough confidence to find happiness. You need to find things about yourself that you're okay with. Build on them. Work on them and play off of them. Make sure to find something because I promise you, if you even find one thing - your glasses, your smile, your ability to make others smile, whatever it is - and remind yourself of that anytime you start to feel unsure, you're going to feel better.
I'm not an expert but I promise you, gaining confidence in yourself and finding a way to feel okay with who you are is one of the most gratifying feelings. It may happen when you're 18, when you're 35, or when you're 60, but when it does, you'll see the beauty in it and why it's so worth it.
Just always remember that you're perfectly imperfect, and you're perfectly you. Love yourself in a time where the world needs everyone to find some self-love. You'll be so much happier in the long run.