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..like today at school i told a kid if i caught him around her again i was gonna kick his *** and shes pregnant with my child(entirely by mistake) and she always seems to throw it up in my face when i get in trouble...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? or have any pointers on how i can make this right again? i miss the old us...i really want that back