When it comes to rapping I place emphasis on two major things and that is lyrical content and flow.
The lyrical content is easy to do if you got at least a couple sparking brain cells and an idea of merit.
Flow is all about pacing your phrases and breathing so that your rap comes across effortless.
I don't know how much musical background you have, but most of the basics boils down to counting 1-2-3-4 in rhythm.
Off timing, break beats, stops, and such put it a lot harder. Busta Rhymes is a prime example of someone
who does his rhymes off beat and fast, but creating a harmonic flow of his own.
Your personal issue is with a habit of speeding up your sentences to where you get off beat
and go against the actual song structure without trying to harmonize or sync with it. You really
have to sit down and listen to a beat long and hard, then start putting your words to it so that
your phrasing and breathing are in sync to it. That is flow.
My background is classically trained music and playing in varage punk -> metal bands. You can
go against a beat like Dave Brubeck or Helmet, but you have to find ways do it more melodically
like Dream Theater.
This isn't your style of rap that pertains specifically to you, but it is a prime example of how flow
works to perfection. Tupac places the phrasing on beat and even when he speeds lyrics he still
finds the beats without effort. It's all about making what you do appear effortless even if you are
working your ass off to get it done.
Here is the other end of the spectrum as Busta still uses a basic 4/4 timing beat, but uses break beat
and speed phrasing on top of it while still maintaining the time. Feel the music and put the lyrics on
top of it and keep trying until you finally feel the words and music blending together and you'll know
what I mean by "Flow".