LYNN ? Lynn dentist Bryan Cousin has seen some dark days in his life, but lately the light is shining down on him.It all started, according to Cousin, a successful Lynn dentist who lived in Marblehead and had a ski house in Vermont, in 2008 when he could not lift his right arm. He went to Dana Farber Cancer Institute and that’s when he was diagnosed with non-Hodgkin’s Lymphoma. Things only got worse from there. As he said Monday in recounting the ordeal, “this is a serious disease.”He had mixed emotions because while he was obviously concerned about the diagnosis, he also did not want to let it defeat him.Still, he began vigorous chemotherapy treatments and continued them for almost three years. Cousin had to stay at the Dana Farber Institute in Boston for three to four weeks at a time, as he was getting up to six treatments a week. The last time he received one, he went into a coma and he also had a toxic reaction to the chemotherapy at one point. That resulted in kidney trouble, which took a physical toll on him.Yet he was steadfast not to let the treatments get him down.”I took it one day at a time,” he said.It wasn’t just Cousin’s health that was in peril. Prior to his illness he had a thriving dental practice in Swampscott, happy patients and a happy family. One by one, however, he lost it all ? his practice, his houses and his marriage. His patients couldn’t wait for him to get better so they had to find a different dentist, resulting in the sale of the practice. He later sold both of his homes.”I was more devastated about losing my practice because I put so much into it,” Cousin said. “It was my passion.”He recalled a time where he was able to do anything, and was involved with the community and deeply committed to his practice.”I used to be extremely busy,” Cousin said, though cancer changed that.Gradually, however, Cousin began feeling better. Today, he says he is cancer free and has a low likelihood of the same cancer returning.Cousin has begun the process of turning his life around. He bought a new practice in Lynn last September, and officially opened last month at 205 Western Ave. He has been involved with the Swampscott Rotary for 35 years and is active in the club again now that his health has improved.”I know I’m lucky to be alive” he said..Cousin, who grew up in Swampscott and graduated from Swampscott High, earned a bachelor’s degree from UMass Amherst and went on to Tufts Dental School.He’s hoping to renovate his new office and is also looking forward to resurrecting the dental assistant program he ran when he had his other practice.As for how he feels?”I’m happy I’m walking around,” he said.