Do i really love him?

My boyfriend and i have been together for 3 years now. When we're together we have a great time. He makes me laugh, and smile and i always have a fun time. But lately, he's been gone for bootcamp and now a school. I miss him terribly and i cry sometimes at the thought of him being gone for so long. This is my longest relationship, and for me its confusing because ive never felt this way toward a guy. I feel so relaxed with him. Like he feels like a best friend. And the emotions and feelings i have towards him feel intense, but im only 19 and i dont want to make any mistakes. How do i know if i really do love him?