Why do I keep having negative thoughts and live in fear

Everytime i be thinking in a negative way, I live in fear of pipo who died and death and keep thinking about them. I get nightmares at night and I feel so scared and again I live without hope so am confused yet i don't have any disease in my life I don't know what caused all this, it has persisted I try to forget and do my best but memory keeps on thinking negatively as if life has stopped. Thank you