How do I know I’m happy?

I’m in a great relationship of almost 9 months, my longest relationship, and most days I’m happy. But there’s days where I realize I have a whole life ahead of me. I have a dream to go to medical school and become a nurse or doctor. My boyfriend plans to stay with his career in management. But what if someone comes a long who I fall in love with, perhaps in college. I feel like I would be throwing away everything I had. I would break his heart and I’d even break his heart if he knew I felt this way right now. I’m scared of this commitment that’s happening so soon. He loves me to death and I’m afraid I’m gonna be with him for a long time just because of the fear of hurting him. I’m young and know there’s limitless things for me out there. Last night I had a dream of a completely different guy and it felt so good to be with someone different. I miss the past of mystery dates and meeting guys and flirting. But then there’s days when I feel fine and happy with the person I’m with. I’m so confused and it’s leaving me with anxiety and guilt. What do I do?