Agree with the others, it's all about maintaining a constant balance between machine and hand speed.
1) Relax your shoulders. Tensing them makes it more difficult to maintain a constant speed.
2) Hopefully you have the machine set into the table for a broad flat surface, and the height of the machine is at your proper ergonomic height (roughly at height of elbow).
3) If you have a speed control and on/off button on your machine (as opposed to using the foot pedal), try that to maintain a constant speed on your machine.
4) Keep practicing!! Don't worry about the design you are trying to stitch, just concentrate on maintaining the stitch length.
Lastly ... it doesn't have to be perfect. It's not unusual to have a slightly smaller stitch length around tighter curves.