How difficult is defamation of character to prove in court?

My former boss fired me without just cause- I live in KY and this is permissible. But, to make matters worse he told my then bf blatant & malicious lies about me- these lies ranged from stealing money from the business to saying I was doing drugs at work and he had called the police after firing me and they found a large amount of drugs in the restroom that were mine! He claimed there was a police report and that I was under surveillance! None of this happened!!!