My boyfriend is leaving

I’m 15 and my boyfriend is 17 soon to be 18, he is graduating in January and will be joining the army immediately and I’m terrified. I really truly love him say what you choose about how young I am but I’ve dreamed about him every day since I was 5. He’s my Prince Charming, pain in the ass but most importantly he’s my best friend. I lost my dad really young and nearly lost my mom a few months ago so the thought of losing my first love is horrific. Any advice on how to calm my anxiety or what to expect?