Sounds like you may have obsessive thoughts based on fear. Its a common symptom of anxiety i had some pretty dark ones at some point much worse than what you are describing.

Take comfort that this particular symptom affect people of a sensitive and good nature (search obsessive thoughts if you do not believe me) however to solve this you must;

a) realize that thoughts are not reality
b) let the thoughts enter your mind and flow out again. DO NOT try to fight them as study's show the more you fear your thoughts and stop them the more the brain will increase the frequency of them. Do not try to rationalize them otherwise the same will occur. Just accept they are there and they are silly and they will go.