It is not possible to identify the cause of low self-esteem for everyone. You form your beliefs about yourself over a long period of time and this process is likely to be affected by a range of different things.
It takes me less than five seconds to condemn a CV because it is not saleable. I help lots of applicants to fine tone their CV’s. Most time they don’t know they are a product. Just like every product on the shelf of a shop with descriptions on their sleeve, your CV is your product description