OP, you are getting some good insight here.
You’re at the stage of your life where the possibilities seem endless and wonderful and away from home. You’re looking outward with excitement. She’s seeing an empty room, an empty bed, an empty chair at the table, a home without your voice. She is already missing you, and that is laced with fear, because it’s not a cozily decorated college dorm room she is seeing, but a full-on, real-life military environment Where Bad Things Can Happen. That nameless imagined void is feeding her fear. Quite normal.
Be kind to her. When she says the negative things, simply respond “I hear you, Mom, and I am sorry you feel that way. If you choose to do X, that will make me sad. I love you.” Rinse and repeat as necessary. Hold your boundaries and opinions while respecting hers.
It is not your fault she is having these feelings and saying these things. She has invested enormous amounts of love and time in you, and all she can see is a gaping hole. She is grieving, in a way, the loss of her child and the way things have been for 18 years. All you can do is reassure her you love her.
Be the perfect kid as a gift to her while you are still in the nest. Take it day by day, but if this is what you feel called to do, keep going.
I know my parents were anxious and worried about me when I went to OCS and then to Spain for duty. This was before email, texts and FaceTime, and in a period where the military was only about 3% women. I was excited, happy and could not wait to launch. As I look back, my mom in particular put on a brave show but said she knew this was what I wanted. I probably could have been more understanding, but that’s hindsight now.
Your mom raised you, so her influence is there in your choice of wanting to serve and hold yourself to a higher standard.
If she asks about certain aspects of your chosen path, respond with facts and offers to look at a web page together.
If you obtain your appointment, if there is a nearby Parents Club, she will be welcomed and supported by a group of people who 100% understand this situation.
Throughout your life, you too will seek to protect and shelter those whom you love. When my mom asked me if I was anywhere near the impact site on 9/11 in the Pentagon, I said “no.” That was not true, but after the fact, with me surviving and ok ( I made sure she didn’t get to see the scratches and dents), there was no need to create pictures in her mind. Sometimes, you get to be the adult around your parents.