Confused about my sexuality?

Okay, well, I’m Sara. I’m 18, and I have a boyfriend. Let’s call him Jack. He’s 26 (I know, bit of an age difference, but it works just fine. It’s not weird to me or anyone else who knows us), and he’s wonderful. I identify as bisexual, and my boyfriend knows and he’s completely fine with it. I met him about 8 months ago, and at the time I only wanted to date girls. I’ve never actually dated a girl, but I was convinced that I didn’t want any more relationships with guys. However, because I told him that I was bisexual, he continually tried to win me over. I fell for him pretty quickly, and we’ve been together for a little over 5 months. He’s wonderful. He’s sweet, and caring, and understands me and he’s always there for me. I really love him a lot. We have had sex and it’s enjoyable. He does turn me on quite a lot.

Lately though, I’ve been really confused again about my sexuality, wondering if this is what I really want. I’ve been thinking “Well what if I would be happier with a girl.” I also may or may not have a pretty big crush on his friend Amanda. She’s 18, too, and bisexual as well, and she’s so beautiful and sweet and funny and she’s a force of personality. She’s great and I’ve sort of developed a crush on her recently. :/ Jack has even said to me “I think Amanda has a crush on you. She’s ALWAYS gushing about you. To me. To other people. CONSTANTLY.” I keep wondering if I’m missing out on what would make me happiest. However, I really do love Jack, and it would kill me to hurt him. He’s always saying how no one has ever made him as happy as I do, and he wants me “for the rest of his days” and everything, and how without me, he’s “an empty shell of a person.” I would feel AWFUL hurting him. I never want to do that, because I really do love him a lot.

I’m really confused, though. I don’t want to break up with Jack, because what if it’s just a phase? Then I’ve destroyed a wonderful relationship and hurt him for no reason. But how can I know if I’d rather be with a girl if I don’t go out and do something about it? I would never cheat on Jack, but I don’t want to break up with him and hurt him like that, especially if I’m not even sure. And what do I do about my crush on his friend Amanda? What do I do? Please please help. I can’t sleep at night because I can’t stop thinking about this. :/