Recording your vocals over a "mastered" beat is not a big deal and is not something that would cause a problem with your vocals not being heard above the track...
Think about it...
People record vocals over songs that are made 100% from samples off of albums... THOSE ARE ALL MASTERED RECORDINGS!
Of course, you ideally want to have all the elements of the beat still "unmixed" when you are recording your vocals... "vocals" are just another instrument in your mix and should be mixed in to the song just like any other instrument... not just laid on top of a backing track that has already been mixed... ideally.
TURN THE "BEAT" DOWN (as was already said)
But "mastering" is the finalstep in the songmaking process.
Basically, when you say the beat was "mastered", you are just saying it was "compressed and eq'd"... It is nothing magic in there that makes "mastering" special. It is just the final step preparing the track for manufacture creating the "master" track.
And, unless you are preparing for a physical manufacture of an album... and unless there are multiple songs... saying you "mastered" something doesn't mean crap. "Mastering" involves creating the track sequence of the album, prepping for manufacture, making all the tracks sound the same apparent level, making the tracks all sound consistent tonally, etc...
Don't get caught up in the word "mastering".... because when most people use the term online in general (who are not professionals who know what they are talking about, that is) they just mean "make the track loud" or "compress the hell out of it and eq it"... not much different from when those same people compress the hell out of and eq all the other tracks in their session.