It's like patty your head and rubbing your belly at the same time!
Practice is the answer, sorry to say.
Adjust your tension if the thread is popping up, usually one click at a time until it stops.
Slow the speed down on your machine if you have that capability, if not, then rest your foot half off the petal so you can't press fully and that may help to slow down the speed.
I have always found it easier to move sideways when fmq, I seem to be able to coordinate the movement easier. So experiment, see what works for you to enable to keep the stitches even. We all have a different coordination system, so what works for one, may not work for another.
Good luck, stay with it, it is worth the effort. Once you master it, you can pick up the speed, so don't feel like you have to be a speed demon at first. Speed comes later. Right now focus on the stitch evenness.