Dolly Martinez , who appeared on "My 600-Lb. Life," recently died ... and TMZ has learned more about her tragic passing. Staci Thurman , Dolly’s mom, tells TMZ … the 30-year-old died of congestive heart failure at a Fort Worth hospital. Dolly had been sick for a while with medical issues ... and we're told she went to the hospital on March 29 with fluid on her heart and lungs. She was put into a medically induced coma and placed on a ventilator the next day -- but never recovered. Staci told us