How do I get my best friend to open his eyes?

Ok, when I was in 9th grade I went out with this guy nathan. To this day I have known him for about 6-7 years. We broke up about more than a year ago but I still had feelings for him and he had for me but we knew it wouldnt work out but we still remained the best of friends.

After I had a massive break up with a boyfriend, nathan was there for me and hung out with me before school and walked me to class. Then later this girl steph ended up sitting next to him where we hung out.they were good friends and later he paid more attention to her. And I got advice that I should be with someone that accepts me for who I am and loves me for me and loves my flaws instead of having to put up with them. When I thought about it nathan came to mind. When I was ready to tell him I think I am still in love with him he beat me to the punch and told me he has a crush on steph and plans to ask her out.

I just told him it was good for him that he has found someone. Then after they started going out drama with her started. She made him choose between me and her since she was jealous that I am his ex and his best friend. He of course chose her and I was left by myself and sad since I still wasnt over my recent ex but I had to deal with it on my own since nathan spent all his time with her. Eventually he ended up talking to me again and we were cool since I understood the feeling he has. Then later he gave her my number and she started txting me. She is flippin annoying. She is always bragging to me about those two. Asking me if I still like him (I do but that is behind me since I have a boyfriend) and she just flipped out on me for telling her I am not in the mood to hear about how far she thinks I have gone with my boyfriend even though we havent even gone pass kissing yet she has already been porked by nathan. And also I am worried for nathan since he is only a junior and her a senior and she told me she wants to get pregnant by the summer which would ruin his life. And she disrespects me when she txts me. I want him to see she is not good for him. How can I do that? Please help me