Statute of limitations for defamation

After over 20 years of excellent reviews as a high school teacher and having just received my Colorado Principal’s Licensure, a former principal made numerous serious false allegations about me to the school district Director of Personnel. When I asked coworkers to write letters of support for me to prove the allegations were false, I was told not to discuss these issues with coworkers or I would be terminated! The allegations were then broadcast about the district, effectively destroying my chances of being hired as an administrator at any school in the district. This same principal made my work environment so unbearable that I was ultimately forced to take early retirement in the fall of 2007. As I am now at retirement age, I have become fully aware of how much permanent financial damage this entire situation did to me. Is it too late for me to seek recourse?