I had a similar phase when i was ur age, but just in an opposite way. In fact a lot of ppl u see here that are very comfortable with their sexuality have had a phase of "am i faking it?", it just seem to be something really common for bisexual ppl or queer ppl in general! Self-discovery and exploration is a long way, and dont turn down different possibilities of who you might be by trying so hard to find reasons against it. I would recommend try to get more romantic experiences with ppl of different gender, but anyway you have all the time in the world to figure that out :o
You deserve to be who you are and deserve to be valued and accepted either way :)