Adapted form a previous post of mine:
In my 40+ years around pool, I've noticed that the biggest difference between the C player and the B player is that "B level" is the first level at which players tend to play with the full range of strokes.
The C players I've watched over the years tend to stick to draw and follow, and also tend to restrict themselves to center and outside english. In other words, they rarely use stun strokes and rarely use inside english, and that's because they have no command of inside english and because stun draw, stun, and stun follow are outside their comfort zones, producing too many missed balls and missed shape. Lots of C players think they play with a full range of storkes, but few actually do.
Study 90 degree rule, and learn how to produce every angle off of the object ball. Learn draw, stun draw, stun, stun follow, and follow. Then learn to produce each of these strokes wit left english, center english and right english at the various speeds of stroke.
If you are a C player, I'll bet the house you are not playing with a complete stroke set, and unless you change this, you may never reach B level. You must do something about it. An instructor may ease your path, but I've seen many learn this on their own.