How to deal with insecurity?

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This is my biggest issue in life, I was avoiding it because I was too afraid to face it but now I am thinking it's really the main root cause of all my problems.

Some story, I was bullied in school and at the same time my parents weren't supporting me either partly because they're facing their own issues too, so I never received any sense of self-worth despite being intelligent enough to be self-aware of what was going on, but somehow that doesn't change what it doesn't change, if that makes sense(because it's only something I don't understand to me and scary).

Later further in school there was another such situation going on, but this time I lost my cool(partly maybe because I wanted to and thought it would solve my problems) and hit the guy with an object, no serious injury caused, but then while I vented my frustration verbally the person didn't have the courage to face up to me, couldn't look up, since that event he always seemed tame around me and talked with guilt+sort of formal friendly/kindness, so then it looked like that person is much weaker then me, I have no feelings of burden from that, it just seems so like he is so behind me in life.

But back to first incident, I never recovered or saw that face of the "that word for crime causer", and I still feel powerless, it's not that I can't fight just, but that my own won't support me and partly, it's truth that these "my" people don't really understand me.
Are all bullies, or anyone who hits someone who is down the same? What could be that cause, it's just something fearful from my perspective although I(my intelligence) understands that they're shrewd inside and the cause is pathetic.

Every time I imagine having a intimate relation with someone I feel like they'll be hurt by similar people and I won't be able to protect them, and that seeking relation is just me running away from insecurity issue(and maybe that's true).