I can’t determine your sexual orientation on the basis of a letter, of course. What I can tell you is this: Some researchers have said that people are on a continuum from homosexuality to heterosexuality. Some people are clearly one or the other. Lots of people lean one way or another. Still others fall in love with a person, regardless of the gender. You are fortunate to live in a time when it is far more permissible to explore one’s sexuality than in prior generations. The down side is that it can be very confusing.
For what it’s worth: I don’t think your feelings about your boyfriend are an indicator of your sexuality. I think you’re just not that into him anymore. As you’ve gotten to know him better, you may have decided that he really isn’t the one for you. Listen to your feelings. Don’t waste his or your own time. Break up kindly and make yourself available to start seeing other people. You’ll figure out who you’re attracted to as you go through the very normal process of self-discovery that goes on at your age.
I wish you well.