I duck taped a little wedge so the speed pedal could only go partway down on my old Singer Genie. Worked well for me. You can either use wood shims or some of the mini post-its (1x2"). But it really depends on how your speed pedal is designed. I know my new Bernina doesn't have a gap between the top & the bottom that's accessible from the outer edge so it wouldn't work for that. Might help if you post a pic (or at least the name of your machine) so we could give more targeted advice.
The one thing that should work regardless of your machine is to make a similarly shaped/height wedge for your left foot. Putting your body in balance will help you have more control over how much pressure you're putting on the pedal.