You've gotten tonnes of good advice so far, but I figured I would chime in, too.
There is no quick fix - as others have said, practice is key. However, my FMQ skills have taken big leaps forward with relatively little effort two times, both due to changes in my quilting equipment.
First, when I switched from a hopping foot to a gliding foot, my quilting suddenly became smoother. Seriously, it was incredible what a difference that small change made. "Breaking" a foot, Leah Day-style, should have the same or similar effect if you don't have a floating option on your machine. Since you say you're experiencing jerkiness, I strongly suggest investigating this option.
Second, when I switched from Machingers to using the Martelli gripper ring to move my fabric, my quilting improved again. I became instantly more capable of keeping my stitch length uniform(ish), and my overall control was better. It was especially noticeable in patterns with straight line segments. I was also able to quilt for longer without my hands getting tired and cramped, so there was more opportunity for practice. Interestingly, I didn't realize I was having such a problem with fatigue and cramping until after I started using the gripper ring.
Those were the big things that helped me. I also found that doodling (creating muscle memory) was valuable, and reducing the maximum speed of my machine so that I could "floor it" all the time and be going at the right speed. This allowed me to focus on my hands, without worrying about what my foot was doing.
Good luck! It sounds like you're on the right track. Keep practicing, and you'll get it!