Believing yourself means having faith in your own capabilities.You are the one , you are unique no one is like you in the World and you are not like anyone. You can do whatever you like to do. Something great that someone did ,you can do that in this world too. No one can stop you to prove yourself. You are the best and created for a purpose don't underestimate yourself, believe in your ability to succeed. Work on your strength and recognize your weaknesses. 

You are not less than others and you are not more than others, the thing is in some aspects you are not like others or less than others  and in some aspects you're more than others .when you look at the side where you are less than others   you get disappointed and i know you rarely look at the side where you are more because it is human tendency that we are not satisfied with what we have even if that is more than others, but if we look at that side we become happy. We should have faith in ourselves, sometimes we doubt ourselves but remember no one is perfect ,other persons also doubts as much as you, but takes the risk anyway. You can be anything but you can't be everything.Everyone can't be the same, everyone's circumstances are different and you should not be alike but the thing which matters is that how much have you developed yourself self and how much are you doing ,see your past and your success (even if it is little )leave the others they are different their  situations are different and they can't be you. What matters that how much you love yourself. 

So. You should not compare yourself with others it will degrade you  comparing yourself with others allows others to drive you. You are too precious.don't underestimate yourself  God has created you with his hands and designed you with his heart . Love yourself it is more important than anyone else love you. Life is too short enjoy it with whatever you have ( not saying that you should not improve yourself) because we don't know what will happen tomorrow. Live in today, something good will happen tomorrow don't spend time in this hope. Discover the things in which you are already good and focus on positive things. 

NOBODY WILL BELIEVE IN YOU UNLESS YOU BELIEVE IN YOURSELF.