Insecure? Helppp please

Lauren • Hi, im very new at this but I needed something to keep up with my menstral cycle & fertility. My name is lauren, I live in Alabama. I am 20 years old & going to college to be a Registered Nurse (RN).
How do you get out of the habit of being scared your going to lose your man/fiance so you are so jealous that you dont want him even talking, looking, or having anything to do with any other women?