If you think its a passing feeling, try self help books, like the "Feeling Good Handbook". Its meant for depression, but it helps deal with unreasonable thoughts.
If its something that doesn't seem like it will pass or you have been dealing with for a while, see your health care provider and ask for help. From a medical diagnosis, treatment depends on the underlying cause.
Treatments range from therapy and drugs for developmental causes, to surgery for physical ailments like tumors - I don't mean to scare you - the last one is unlikely