The occurrence of serious side effects with Gilenya is actually very low. Most people tolerate it well. It's the only DMD I've used that caused me to have no side effects aside from low WBC. Blood is monitored every three months to ensure they don't get too low. Liver function also monitored.
What did your doctor say about your heart rate?
A couple of years ago I lasted just 19 days on Gilenya after switching from Avonex, which I then returned to. Gilenya seemed to have adverse effects on my heart, and I was told I looked like my soul had left my body. No DMD is a free ride, but with several available you can find one that you can be more comfortable with.