There is no way to let her know you are going to be OK because the odds are that you will not be OK. You could very easily be killed in action. My advice would be to find something better to do with your life other than wasting it on the Army and not let others dictate what you do with it. Daddy or not.
Put her in your will.
Number One Advice : DON'T DIE
Your girlfriend has suffered so much loss that she's become terrified of loss in general. If you were to give up the Army, she'd be afraid of losing you to any one of a million other things that happen in life. It sounds like she'd really benefit from finding a counselor who can help her work through her fear.
Shes going to die too , offer that as consolation
There is NOTHING you can do to calm her fears. A couple of the alpha males in her life left her and you are about to put yourself on harms way. I'm not saying don't do it. I'm telling you thats how she feels and you should factor that in to your decision making. You are going to hurt her and worry her if you do this. Theres just no way round that.
Keep your head down and stay safe.
Make sure that your insurance has her listed as the beneficiary, make sure she is in your will-see above-make sure she hass all the VA phone numbers to call for benefits, call her as often as you can, talk with your superiors, buddies and some older vet's about what they have done for their significant others-they may tell you some things that they wish they had done, and let her know you love her.
That and read all the suggestions everyone leaves here.