Re: What should my next move be. (Very important)
IMO if you really want to dig yourself out of this hole you might have dug CALL. I don't know who this rapper (?) is, but if I was even regionally well known and was big enough to have an manager, there is a good chance I'd be too busy for an "opps" text from someone I don't know.
Either way you aren't going to get out of looking like a little bit of a spaz. I'd say if you CALL and get to talk to her or even her manager and in a couple sentences say "Hey, I got a bit over excited and rashly sent out the first thing I could find. I've made something I think really fits the bill and have sent over. I'd be grateful if you could find the time to listen." if this Becky G is someone worth working with she would listen to the new beat.