Are you practicing on an actual quilt or sample sandwiches? Small samples about 15 to 18" square are great to start with. Start on paper and just repeat drawing circles and curves. It is a hand - memory skill which takes practice. No one is born with this skill. It is learned.
Draw lines right on the fabric and work to stay inside the lines just like learning to write when we were kids.
Once you feel okay with how your curves look on paper go to the machine and practice there. SLOW YOUR SPEED DOWN. No need for a heavy foot. As you get comfortable just moving the fabric around start doing loops and smaller curves, waves, figure eights are great for practicing. There are many places to get ideas for quilting patterns to practice on. do a search.
Slow, relax, breath, shoulders down.
Take a few slow small stitches as you start and stop. Just like driving, you don't start out at 60 mph.
Practice doing a straight line and then put a loop it one direction then the opposite.
The one thing I learned is to have paper to practice any design on before going to the machine. Any paper will do even old news paper. It is all to get the hand memory going.
Hope some of this stuff helps you to keep going. I've been there so I understand. It can be discouraging but don't give up.
peace