EDIT: What I did was get used to a specific sensitivity and then get good with those certain settings. It's mostly preference- someone else's sensitivity may not fit your play-style. Also, what's your IGN? You could PM it to me if you'd like.
But 18 is like slow motion how can
you even move from the bullets your opponents is spraying at you
It's not THAT slow. Just get used to a sensitivity that you're comfortable with. You can't become instantly good. "The greats weren't great because they were born to paint; the greats were great because they paint a lot" - Macklemore