Self-Doubting Lurker, Bisexual, too many questions

So, I'm part of my school's GSA club. We're having a presentation at Morning Meeting at what it's like to be part of the LGBTQQ... (there's more letters I know) at our school. So all these kids have these GREAT experiences, like bringing their gf to prom, being able to say who they are etc. Our school's a pretty accepting environment. But my experience isn't always positive, I mean yea my friends are very accepting, most of them are gay or fall under one of those letters. But other kids at school have said things like, "you're 14 and already bi?", "it's probably just a phase." etc. Even my parents think it's just a phase and nothing serious, they don't mean harm they just think that I'm trying to figure out who I am and I'll make a choice between straight and gay when I'm older. But the presentation was called off, and I've just been left with this written piece about how I feel and how other people have reacted to it. So now I'm just left with all these questions in my head.

What if they're right? I like girls, and I like guys- and that gives people enough reason to believe that I'm just looking for attention. I feel like I've only just discovered these feelings, so maybe I chose it instead of being born that way... so does that mean I'm not bisexual? When people ask me if I've "always felt this way" I don't know the answer. It makes me feel REALLY self-conscious. Do you guys have any of the same feelings?