I felt exactly like this most of my life. Embarrassed. Embarrassed to exist, cringing at my own existance. Feeling silly, stupid, worthless, ashamed, guilty. Wishing I could disappear. This caused no self esteem and extreme shyness, I mean, who would want to be friends with me?
I dont feel like that now. No way.

I now know through much research that it was, in my case, upbringing that caused it.
Mother was a cold, unemotional creature who made sure I knew that everything wrong in her life was actually MY fault. That I was a burden, and nothing but trouble and how much better her life would have been without me.
So she 'taught' me to despise myself. And boy, did I HATE myself.

It might be different for you.