You dont have ''low self esteem'', you re just genetically, psychologically weak and there is nothing you can do about it. Some people are strong minded, others are weak minded, as simple as that. Some people are raised, conditionned to be strong and kill instead of being killed, others are conditionned to be weak, to get killed.
You can try anti depressants , getting big, get money to improve your condition tho. But in the end life is unfair, some are born or grow stronger than others. And the strong feed himself with the weak.
To be honnest improve what you can, but it's pretty much obvious you will never change what you truly, genetically, biologically, are. You have to live with it, adapt to survive. Some weak minded people can live long lives if they pair with someone stronger. I guess all you re left with is to look at people weaker than you instead of lookind at people stronger than you to feel better.