My jealousy is sickening

I’m a jealous girlfriend. Very insecure and I can’t pinpoint why. I literally think my man is perfect and I try so hard to be perfect. I am intimidated by his ex which they have a child with. She’s skinny and tall. I’m short and well muscular not fat but thicker. I know I am good looking I just can’t pinpoint my insecurity and where it comes from

My sisters told me I’m insecure with him and I don’t fully understand what that means?