Can I sue SunLife if I signed a Settlement & Release Agreement?

I hope I can still sue SunLife, despite having settled in spring 2017 for an unreasonable low amount due to fear of dying before they pay a reasonable amount. The reason I settled was that I feared of dying from cancer before they would finally start paying LTD. Before settling with them for a fraction of what they owed me, I send them multiple faxes with each new medical test result every few months, always pointing out that “I hope they pay before I die”, explaining that I believe the symptoms were caused by cancer. History: In 2015 my health declined rapidly. I immediately suspected cancer. In fall 2015 I filed for Short Term Disability with SunLife. I had lots of medical test done, and before end of 2015 two doctors independently of each other suspected cancer. But in January 2016 SunLife refused to pay LTD. Fall 2016 cancer was diagnosed, but still SunLife refused to pay. Spring 2017 SunLife call me with a settlement offer, saying “considering the cost of litigation, we offer you a settlement” and “we don’t see any reason that you can’t work”. I settled for an unreasonable low $6k out of fear of dying before they offer a reasonable amount. I could not do this to my wife and son, die and than let them go to court against SunLife. I thought, at least get a little money now, as long as I am still alive. It would help my wife a little bit right then, since she had to work 2 jobs due to my inability to work. My wife would not have the strength to go to court once I am dead. She already drove me to all doctors appointments, emergency rooms, MRIs, Pet Scans, Cancer Hospitals, always sitting in the corner of the exam room with tears in her eyes. Only in winter 2018 it was diagnosed how this cancer caused the severe symptoms that caused total disability since 2015, it was nightly panic attacks with total collapses from fear of dying of cancer and fear of suffering from cancer treatments. The fear of treatment side effects resulted in me delaying treatment by 2 years until spring 2018. It was clear at that time I would die very likely pretty soon if I don’t get surgery. I needed psychotherapy and Valium before I was able to agree to surgery. The panic attacks caused extreme lack of sleep, and I could not function reliably during the day. As a side note, in Japan each year 10thousand people die of panic attacks in their sleep. It felt like some of my panic attacks almost killed me the same way. Heart blood vessels contract due to fear until the heart stops. Well known and researched in Japan, almost unknown in the rest of the world. After SunLife in spring 2017 offered me $6k. I consulted a lawyer, who told me it would cost $60k to go to court, but what SunLife owed me at that point in time was less than that, so no lawyer would take my case. And obviously self representing in court is probably a guarantee to loose. I thought this case would end up with a multimillion dollar settlement, but I guess that would have required me to die, and then my wife going to court against SunLife.