- Being INTP. We want to base our confidence on information and knowledge, and since we are honest about what we do not know, often this hamstrings out confidence compared to other people. We cannot easily lie or pretend to ourselves.
- Family. My dad was alcoholic and so all my life the "rules" of relationships were really screwy, I was basically told all my perceptions were wrong (even if I believed they were right), and I just couldn't trust people to be fair or honest. The inherent elements of denial that exist in screwed-up family dynamics really wreaked havoc with my self-assertiveness and self-confidence; basically, I learned instead of believing in myself to just avoid any potential conflicts. I also felt that I was powerless to make changes in my own life, especially in relationships.
- Religion. Not to bust on religion, but it made me doubt all of my instincts and perceptions in the culture I was part of. I was told that I had to accept the revelational truth AND the doctrine spawning from it, and that other ideas I had were either sinful or selfish.
- Lack of life experience. I just did not have experience to speak authoritatively on anything and trust my judgment. All my ideas were theoretical... and thus potentially wrong.
I think life experience will definitely give you confidence. The more you have experienced something and/or acquired information about it and processed it all, the more confident you will feel in your assessment.
You also probably also start to accept that it's okay to promote yourself or take a positive spin on something (rather than a neutral/skeptical one), since positive feelings beget positive action and potential success, where incessant skepticism often derails your endeavors. There are parts of life that are not meant to be analyzed per se, instead they are meant to be CREATED by your attitude and choices and dreams and goals... subjective behaviors, not objective ones.
You might also learn to evaluate relationships (and even build them) via other means than critical analysis. There are things in life meant to be enjoyed rather than constantly deconstructed.
All of this accrued knowledge builds confidence.
Only instigate battles worth fighting.