okay, I see the situation clearer now. Your b/f had the option to go but didn't want to for his own reasons. I'm not sure if he knows the first part of the trip is just the two of you. I would tell him, but my guess is that you won't. But, no matter, you just have to make sure you don't step out of line( since you mentioned that " I don't trust myself to go and not do something horrible").
Since you're not married, you shouldn't be too hard on yourself. But if you think you're definitely gonna go for this guy, you really should break up with your b/f first. He deserves that much, at the very least.
Go for the trip, take in a different perspective on both your relationships and find out what you really want. This is what dating is all about right? ......... just 'don't cheat', that's all. I truly hope it works out for you