Why do i get sad when i think of my gf?

I love her dearly and we get along fine...sometimes but even when we are getting along when i sit and think about her i feel depressed and i worry shes gonna leave me...i freak out on every guy she talks to and shes pregnant with my child...i never wanna lose her or the baby but i fear what im doing and how i treat her is only gonna push her away from me :( anyone know whats going on?