Most Helpful Girl
The quickest way to overcome being selfish and entitled is to find a wonderful guy you love and adore, only to have another girl disrespect you and your relationship by luring him away from you, emotionally. However, you seem to understand your faults and shortcomings, and seem to genuinely want to change your behavior; and to do that, you really must give him up entirely. Butt out of their relationship, do not answer his texts. Whatever has happened to you in the past has no bearing on your choices now, or in the future. You DO know right from wrong, so take the moral high ground from this point, on. You wouldn't want a guy that is committed to someone else, but cheats with you... or vice-versa. And to be honest,16 is REALLY YOUNG to be worried about these things! You should be enjoying your independence, now... worry about relationship stuff after high school, when there will be so much more time for that.
You seem unusually smart, self-aware, and highly evolved for a girl of your age. I have little doubt that your life will turn out just fine. :)