Advice Needed Please

I'm new to this website, so I hope I'm doing this right :)
This is a REALLY long story, so please only help if you can bother to read the whole story and help me
I'll start by stating the basic facts (like who the people I am referring to are)
I am 16, and gay. I am in an online relationship with a guy who is 17 (name is Zack) Another person I am going to refer to in this is 14, his name is Marc, and as far as I know, is straight.
Alright so here's the deal: About 2 weeks or so ago, I started to develop a bit of a crush on Marc. I don't know him personally enough to start a random conversation with him, but I have seen him say things on Facebook, and he seems like an awesome kid that I would get along with easily. Up until a few days ago, I was fine with just leaving him alone, but a few nights ago, I was talking to one of my friends about having a crush on a kid while being in a relationship. When he asked me how I really felt about Marc and my boyfriend Zack, I realized how much I really do like Marc. What started as a simple crush has evolved into a much stronger one, and I'm finding myself thinking about Marc much more often than Zack. Marc has a girlfriend, and as stated, as far as I know, he is straight. I tried to convince myself to forget about Marc, since I knew nothing would happen between us. After convincing myself that, I went into a deep 1 or 2 day depression for no reason. That's when I decided that I might try to just become friends with Marc, in hopes that my stronger feelings for him would fade. Oh boy, was I wrong.
Yesterday, my crush on him returned, and I couldn't stop thinking about him. Today, in study hall, he showed up (unexpected to me) and he sat right next to me. My stomach turned upside down and I felt sick to the stomach with anxiety and nervousness that someone I liked so much was sitting right next to me. What's even worse is that I noticed myself PURPOSELY listening to bands that he loves, because I wanted to know what type of stuff he likes the most. I feel like I'm becoming obsessed with Marc, and I'm really starting to get worried :/

That's it for the story.....here's my question. What the hell do I do? I have a boyfriend who I am starting to like less and less because I am gaining a like on a person who I'll never be with! I don't want to lose Zack, and yet I want to be with Marc. I've given up trying to figure this out, because every decision I've made so far on the matter has made it worse in the end. There probably isn't too much that you guys can do for me, because I think I already know what to do. I just need someone to guide me, regardless.....I'm feeling horrible.