What do I do now?

This was why the golden rule was invented, NEVER date your best friends boyfriends or girlfriends, it never ends well. One thing you could do is play dumb make up things guessing why she's mad, don't blow it by letting her know when that might not even be it. And if any one spilled the beans it was your ex, who else but you knew and you didn't tell anybody. You had it set up you just forgot how strong gossip and rumors are. But if worst comes to worst just admit your sin and get it over with, don't loose your bff over something like this.

There isn't really a way to fix it. Just be honest with her, tell her your feelings and how you would do anything for you (obviously from the story). But the thing is you didn't think about her feelings, you were willing to hurt her to get her. That ain't right man. But back to the point, tell her every detail of what you intended, don't leave anything out.

for the future man, don't try to create love, let it create itself.