How can i be sure my bf doesn't go back to his ex again?

They were together for like 9 months on and off. they broke up the last time new years day and my friend introduced me to him. i was attracted to him right away. we hit if off and had a great connection but stupidly i sent her a myspace message telling her that i was so happy to be taking her place. then next thing I know, he dumps me to go back to her. Then once again, 2 weeks later he came back because they could not work things out. i again sent her some messages and some texts and called cuz i am so scared he still wants her and I don't want to be a rebound gf. i think I have fixed it to where he doesn't even wnat to be friends with her but how can i be sure?