Low self esteem issues can be caused by different factors. What you're experiencing may not be as a result of a single trigger but several different ones. Adlerian psychology would look at birth order and the way you were parented. For example, are you the only child, the oldest one, etc. Relational dynamics within the family unit will also be examined to determine possible causes behind your feelings of worthlessness.
Freudian psychology would look at possible past trauma issues that you may have repressed growing up. For example, if you were put down by a teacher, parent or another authority figure that you were looking up to or were in their care. Existential psychology would look at your desires for self actualization and how you're meeting your life goals. Some individuals experience what is known as the "impostor syndrome."
This is mainly evident in one's school or work environment. You feel as though you're an impostor and not capable or knowledgeable and that others are being fooled by your accomplishments. Of course, this is not correct. You're merely putting yourself down. If you tend to be a type A personality, you may be evoking the low self esteem because of your higher expectations of yourself and being easily disappointed when things don't work out for you right away.
Depression and anxiety can also trigger low self esteem/feelings of worthlessness. Negative internal dialog (what you think and tell yourself) is accepted by the brain as truth (even if it is not) since, the brain can't distinguish b/w true/false statements as they apply in reality. Negative self talk can be seen as a form of autosuggestion/self hypnosis. You may not even be aware that you're engaging in this.
How others treat you and how you relate to them can also cause esteem issues. You'd want to be mindful of whether or not you may be adopting another person's self esteem that you perceive subconsciously as your own. Try to see what individuals you associate with and how they make you feel. In an online Q & A platform the information may only touch on the surface of your inquiry. There are ways to determine if trauma/past events are the cause of it through hypnotherapy or self exploration therapy. Jotting down in a journal your internal self talk can show you patterns regarding what you hold as true about yourself. Thoughts do not always reflect the reality; and you do not have to believe everything you think of yourself. Reframing negative thoughts/self talk to positive or at least neutral statements can condition the brain to accept those as truth.
Each person has their own level of self worth. Comparing oneself to others can cause feelings of inferiority especially if you feel that you should be in a different place of your life at this time.Lack of closeness to others may be a defense mechanism. It can also apply to you feeling unable to voice your opinion.
It may also be caused by different disorders such as schizoaffective disorder.
Feel free to respond as needed so I can assist you better. These are just some ideas to throw light upon what you're dealing with.