You are getting lots of good advice throughout this thread. I feel strongly that you should seek out a certified billiard instructor and let them analyze you as you shoot a variety of shots. I have been playing pool for over 20+ years and was stuck in my game, which was inconsistent at best. I was a 6 in apa 8-ball with a 30% win percentage (after personally following the advice i am going to give you, i am still a 6 in 8-ball, but now at 90% win percentage, currently ranked 4th in my league). My game has significantly improved, my practice has purpose & I shoot with a lot more confidence.
In September, my wife sent me to Tom Simpson's pool school (3 day intensive). It was expensive, but totally worth it. Shot mechanics are much more complicated than you think (or at leas than I thought). There are a ton of mechanics involved; shoulder, elbow, wrist, eye movement, stance, approach, bridge, hand placement, head, backstroke, forward stroke, transition between back & forward stroke, grip, follow through, levelness of the cue, etc. All complicated by different scenarios - close to the rail, on the rail, english, follow, draw, long shots, close shots, neer a ball, jacked up over a ball, etc.
You need someone QUALIFIED to video you and fine tune all of these things. Everyone is different & different things will work for different people.
Once you receive your personalized advice, you then need to practice - alot. One piece of advice that worked for me was from instructor Mark Finkelstein (he's one of the instructors that teaches at Tom's "beat people with a stick" clinic. Teaching is what Mark does for a living. He is awesome & out of new York). He advocates shooting 100 straight in shots every day (long ones, corner to corner) - paying strict attention to fundamentals. Your shots should have purpose (straight in stop shots one day, follow the next,etc.). Do this after you receive your instruction as you do not want to do this prior and just continue to practice with habits that may not be the best.
There are some excellent videos and books out there, I own them all. I am also a member of the accu stats video of the month club and I watch and study a lot of pros. You will find a ton of variation in the pros (and many with habits that go against what you may see in videos and books). But the pros hone their craft over millions of shots and tons of hours. They can overcome habits they develop over time simply by putting enormous amounts of time in (more than the average casual player). I spent a lot of time reading, watching and practicing, but nothing had the impact as when I received personalized analysis and instruction from certified billiards instructors (literally three days that changed my pool life - sounds dramatic, but true).
If you are like me, you put too much time into the sport you love to waste your time - practicing for hours, days, months and not improving to your satisfaction. Seek out a certified instructor - not neccessarilly a pro player - but a certified billiard instructor. Someone in this forum can point you in The right direction. You won't regret it, I promise!