Many things i'd tell you right now, (for me is the truth) but generally they are "just" my opinion.
As a very confident person i can tell, all you gotta do is to observe yourself from another perspective.
You need to start to believe in yourself. You need to become aware of the power of your brain. Your mind. You need to start to create a character for yourself from the scratch. A character with self-structure. Believe in yourself, have faith in yourself and become sure that you can do. And you are unique in your own way. This is the first thing you must swallow. (everyone has their own unique appearance and everyone has their own unique abilities) therefore,
(Appearance): You shouldn't be unconfident about how you look. Because you have your own special appearance. The ones that no one else have but you. (colors, sizes, numbers), those are the least important for achievements but also they make you to be you and you must embrace it and see the brightness you own within). You'll be amazed of the impressions you will catch after seeing yourself differently. Flaws are part of us. Nobody's perfect, study your flaws and powers and accept them all. Then bring the best out of yourself.
(Abilities): When it comes to abilities, anything that comes through you and your life from the smallest thing to the biggest ones, you must know that life is limitless and our capacity of achieving is unlimited. You can do whatever you want to do, and you can reach any goal you set. Small and big, short-term and long-term. What you need is (Consistency) (Faith) (Self-Believe) (Courage) (Positiveness) (Focus) and some (Organization) including (Passion) reaching these factors are easier than you think, you must ask yourself why wouldn't you believe in yourself ? If you do something and you fail, you must know, success comes with series of failures. That's how we gain experience. And experience guarantees our improvement. So at the end of the day, you lost nothing but earn points. If people around try to take you down, well you must know these kind of people will always be there. You must listen to yourself and your heart. You know who are you and where you going. When you know that, doesn't matter what they say, you know what you are capable of and you're going it shine it over people just like a sunrise. What you need to do is to believe in yourself. To know that you have nothing less than any other people. Even those who you would personally make an example of as very successful people. You need to approach, you must put yourself in the situation and keep yourself motivated and never look back. Set goals for yourself, as you reach them, you'll feel better each time. Never get tire of repeating something. Remember,"It only has to work once" when it happens, you'll enjoy of what you've achieved, learnt or succeed. And there's a fine line between selfishness and self confidence. Make sure to not confuse them.