Many people make mistakes in their lifetime.
Not helpful. No purpose for this sentence.
A mistake I made was not taking high school seriously and not doing the best I could. ---I think you can discuss a specific class or semester, not the whole high school career.
When I started high school I had all honors classes.----- if this is true, do not be so unfair to yourself as to say you 'did not take school seriously.' You did sometimes!
I don' think you say "I" too much, but you dump on yourself too much... "Summer came and I was out partying with friends almost every night. " this is not fair. You make yourself seem so bad.
Write an essay about an aspect of your personality that is like a personal demon.
For example, I think my ego has often caused me to defeat my own purposes. I could write an interesting essay about the way I thought about things and the result it had... write an essay about something about your personality that caused a mistake and how you can turn that part of your personality into a strength instead.
:-)