I connect to that feeling you have of thinking 'why the heck am i doing this, i'm a logical, intelligent person but i resort to this?!'. But i'm not sure it really relates to our rational side, so it is invalidating to shame ourselves in this way time and time again.
It sounds as if in this post you have answered your own question. It sounds like you are very unhappy and are not even sure what happiness is anymore or how to achieve it (this i relate to!). And since you have always been able to get yourself to the top of the pile despite all adversities you are confused as to why you can't 'achieve' this as well. It sounds as though through-out your life you have constantly been trying to prove yourself to everyone around you, especially your family. It sounds as though your family are quite critical.
I hope i've not over-stepped the mark in this post. I don't mean it to sound like i'm saying it's simple just fix this and that and you'll be fine...because self harm and all it entails is a complex issue. If it were that simple and logical to work out believe me the vast majority of us would not be here!! Have you ever thought about talking to a therapist about any of this? I know that might sound like the cliche answer to everything but i have found it very useful despite my hesitations...and i think there is a lot about it that might be helpful for you too.
These scars are a reminder to me of what I want everyone to forget. How do you deal with this? Do you hope to meet someone from the same situation who understands where you're coming from? Does cutting and burning ever stop being the one thing that makes you feel comfortable?
I wish to God that this sadness leave. I don't know what I can do to get rid of it, and I don't want to depend on someone else for my happiness, since that would be fleeting. . .
Your past is part of you, you can't forget it no matter how hard you try but it doesn't define you. It is not you. I think acceptance of your past would really help you deal with all of this.

I say all of this as if i know anything, i'm still struggling to even admit to myself self harm is a problem! But i do hope that you allow yourself the opportunity to find happiness and to connect with someone again.