What have I got myself into?

I don't want to come off as a whiny bitch, but I can't talk about this with anyone in real life because of reasons you will soon find out.

I got a job at Taco Bell about three months ago. There is a girl there that is the coolest girl I've ever known plus she's also really hot. I'm 17. She's 20. She has a boyfriend, who also works at Taco Bell, who is 25. There was a rumor started at work that she and I liked each other. He got all mad and said he was going to make my life "a living hell at Taco Bell and make me want to quit." At the time I didn't like her but it has only progressed since then. I like her a shitload now. She also likes me as I found out today.

We had become really good friends and we ended up exchanging cell phone numbers. We texted each other a lot and today I hung out with her. Apparently, last night she had a dream that we kissed and she said that she was hoping to reenact her dream at some point today and I just put two and two together and we ended up making out not 30 minutes ago. Now, if her boyfriend found out about this I can kiss my sorry ass goodbye. He's 8 years older than me and he's bigger. I'm going to start working out again, not just for this specific reason, I've been meaning to for a while now.

She has been debating on whether or not to break up with him for a month now and I came along and, to her, all signs lead to breakup. He's a big jackass, but if I was in his shoes and some punk kid made out with my girlfriend I'd be pissed off. Any advice?