Hey dear.
I've learned that basically the source of this kind of pain comes from the fact that a person thinks that s/he's "not good enough." This source causes people to do many things that they wouldn't do if they fully accepted themselves as they are. It manifests itself in many ways. For example, here are a few off the top of my head:
workaholism - you work at every opportunity to try and alleviate this feeling "I'm not worth it" and to prove to yourself and others that you are complete perfectionism - you try to be perfect at everything you do to alleviate the nagging sense there is something wrong with you; you think that being perfect will finally solve the dilemma of you being incomplete anorexia / body dysmorphia - both are a result of believing yourself to not be enough; you believe that something about your body is making you incomplete
There's probably more that you can come up with yourself.
Another way it manifests itself is how you react to something negative that happens to you. If someone embarrasses you, for example, that is something that affects your reputation and sense of identity. You become angry at that person because "how dare they" poke at your fragile identity. The reason they aren't allowed to poke around at you is because there is some insecurity there. The insecurity is that deep down you feel that you aren't enough. (BTW, I'm not singling you out. When I say "you" I speak in general since this thing is extremely common.)
When did this all begin? Well, part of the problem is that from early years, our parents and/or society instilled in us the idea that we aren't good enough unless we prove ourselves. They wanted us to be good at academics, sports, etc. and they would get disappointed with us if we didn't do well, i.e., they would make us feel guilty for it. I think that to the degree that one's parents or society made someone feel guilty about that sort of thing, that is the degree to which one feels incomplete later in life. For some reason, parents & society don't really trust that children will do well in life without punishing them / making them feel guilt at some point. Of course, there are kids that don't buy into all the guilt trip BS from their parents, and they end up doing great emotionally later on because of it.
The way to fix the problem is to change the thinking patterns that you are used to. You need to be able to transition from the thought "I'm not enough - I need XXXXX" to "I'm enough." I'M ENOUGH is a powerful belief that will set you free. When you know you're enough, you don't need to prove it with any kind of actions. You don't need to prove it with your reputation or any external displays. Anything that people do to you doesn't affect your self-worth; there's no need to ponder over what people might think about you. You are your own being and you know that you respect yourself and that's all that matters. You don't take anything personally.
The interesting thing is that when you do things like the bullet points above, you are actually reinforcing the belief that you aren't enough because you are doing that thing to cover up the feeling. If any girls reading this use makeup as a way to alleviate the feeling of incompleteness, I would suggest you use that makeup to write on your mirror in large letters, I'M ENOUGH. Hang some posters of I'M ENOUGH in your room to remind you of it. Then you must remind yourself of this every morning, day, and night, and every situation until you know that it is indeed true. Hope this helps.