Just because you are a good, kind and caring person, doesn't mean you are going to be respected for it.

You are feeling the pangs of being rejected right now. Yet, you are not recognizing that this person is much too busy getting high and he simply doesn't care the way you had hoped he would. The reason you hate this other girl is because you feel she is getting what you deserved for how you put up with him and cared. What you are not recognizing is that this guy is NOT capable of giving you what you wanted, and he will toss this girl the same way. She is another addict and she joins him in getting high, his priority is getting high, THAT is what he cares about THE MOST.

That is how your parents were too. It's extremely hard to emotionally deal with individuals that are driven by their addictions. Hating this girl he is with isn't going to solve your hurt, she is just another addict that allows him to engage and that's really what he wants.